Commentaries on Rashi wonder about something...

Philo, a Jewish Greek commentator, mystic and philosopher from around 5 BCE, problematizes any understanding Adam and Eve may have gained from simply eating a fruit. In his view, understanding is a process not an automatic acquisition.
Bahya ben Asher ben Hlava, expounds da’at to mean want and free will.

Ramban 2:9:
And the tree of the knowledge of good and evil. Radak has said that the fruit thereof caused those who ate it to have a desire for sexual intercourse, and therefore Adam and Eve covered their nakedness after they ate of it [the fruit]... But in my opinion this interpretation is not correct since the serpent said, And ye shall be as ‘Elohim,’ knowing good and evil. And if you will say that the serpent lied to her, now [Scripture itself attests to the truth of his statement in the verse], And the Eternal G-d said, ‘Behold man has become like one of us knowing good and evil.’ And the Rabbis have already said: “Three stated the truth and perished from the world, and these are: the serpent, the spies (from the desert), and Doeg the Edomite (in Shmuel).
The proper interpretation appears to me to be that man’s original nature was such that he did whatever was proper for him to do naturally, just as the heavens and all their hosts do, “faithful workers whose work is truth, and who do not change from their prescribed course,” and in whose deeds there is no love or hatred. Now it was the fruit of this tree that gave rise to will and desire, that those who ate it should choose a thing or its opposite, for good or for evil. This is why it was called ‘etz hada’ath’ (the tree of the knowledge) of good and evil, for da’ath in our language is used to express will...or desire...
Now at that time sexual intercourse between Adam and his wife was not a matter of desire; instead, at the time of begetting offspring they came together and propagated. Therefore all the limbs were, in their eyes, as the face and hands, and they were not ashamed of them. But after he ate of the fruit of the tree of knowledge, he possessed the power of choice; he could now willingly do evil or good to himself or to others. This, on the one hand, is a godlike attribute; but as far as man is concerned, it is bad because through it, he has a will and desire.

Rambam: Guide for the Perplexed (Morei Nevuchim): Maimonides observes that the bible says that Adam and Eve knew that they were naked after eating the fruit. Although they had previously known this empirically , they now had a new perspective on nakedness that came from the da’at, the deeper meaning. What Adam did not perceive as wrong only moments before, he now knew was wrong.

Nechama Leibowitz:
Another possibility is that the tree gave the eater a comprehension of affliction. The original intention, had man not eaten from the tree, was for man to know good through the keeping of G-d’s commands and evil, or affliction, through transgression.

So why prohibit?

R' Shimshon Refael Hirsch contends that the purpose of the tree was in order to teach Adam a lesson. This lesson was for Adam not to rely on his own interpretations in order to determine whether something was to his benefit or not, but to rely on G-d’s guidance alone. Therefore, the purpose of placing the tree and subsequent prohibition according to Hirsch, was in order to demonstrate to man the dichotomy of something being visually appealing but forbidden at the same time. This is the ultimate demonstration of acquiescence to the will of G-d.
Hirsch further suggests that within the prohibition also lay an overt message to Adam that it would not be up to him to determine what is considered good or evil based on his own perceptions or thought processes. Rather, Adam must look to the guidance of G-d in order to understand what is good or evil in His eyes. Only then would Adam be able to accomplish the purpose for his creation and merit the paradise that God had given him. Without G-d’s guidance, and left to his own machinations, Adam would likely try to justify his actions, no matter how evil; more so after partaking of the forbidden fruit, as is evidenced by his own attempt to offer an excuse for his actions by stating that “the woman... gave me of the tree,” attempting to suggest that perhaps once detached, the fruit would be allowed.

Nechama Leibowitz helps us understand:
Ramban posits that man’s purpose in the world is to rule over it and not sit idly, complacent with his place in the earth. Man must utilize the resources the earth offers him to their full potential in order to extract from the earth all of its riches to his benefit. Indeed, it is not man’s role to be a spectator, but an active participant in his destiny, since free will, in some capacity, was clearly built into his makeup. How then can man be told to conquer the earth and simultaneously be told that his freedom to do so is limited? Benno Jacob suggests, therefore, that G-d enacted this prohibition for just that purpose: to teach Adam that he is not unlimited, he is not a g-d to do all that he sees fit to do. While Adam may be a master of his own domain, there is a master over him, and while he may be at liberty to do a great many things, there are some things he is not meant to do. This is the first, and perhaps, most important rule Adam needed to learn

Before and After Eating--
There is no question that a qualitative difference existed in Adam before and after the sin. Was this difference purely an intellectual one? If the sin was considered a downfall for Adam, and thus rendered him less than perfect, might it be said that prior to the sin he was perfect?

Rambam and Free Will:
Rambam takes issue with those who declare free will a byproduct of eating. Rambam looks at two ideas to back up his assertion. His first proof was that man was created “in the image” of G-d. This meant being flawless in intellect as well as physicality. Second, the very idea that man was given a prohibition against eating from the tree implies free will. What did change, however, is man’s flawlessness.

In other words the evil inclination became intrinsic. Note before the sin the snake had to convince Chava to eat from the tree. The evil inclination as embodied by the snake was an external force.

Rambam in Moreh Nevuchim (Guide to the Perplexed) views the pre-eating state of existence of man as higher or more spiritually close to G-d then the post eating state. In his view, man’s status was lowered once he had the choice for sin and, of course, followed through. Man possessed the most important part of intellect before the eating of the tree. The verse states the “tree of knowledge of good and bad” (tov ve rah), and not “truth and falsehood” (emet ve sheker). This is important in Rambam’s view, as it demonstrates that man had the ability to discern between truth and falsehood before eating, but not good and bad. The distinction is that intellect is able to tell the difference between truth and falsehood, whereas it is man’s perceptive attributes that tell the difference between good and bad. Man had no aesthetic sense. Hence, prior to eating man was unembarrassed to be naked. Man’s consequence for not heeding the prohibition was that his original understanding between truth and falsehood became less acute. The purpose of man’s existence before and after eating changed. Man’s goal now is to separate the good from the bad, and not the truth from the falsehood.