Though the creation had brought about a means where matter could possess increased light, that quantity of light was limited by the fact that all of creation was made to be acted upon, not to act for itself. This changed with the fall. The fall brought about an environment where living things could be aware of good and evil, and could therefore choose to acquire more light than would otherwise be possible.
When Adam and Eve ate the fruit of knowledge of good and evil, they opened a channel of perception that was previously closed to them. They gained both a basic understanding of good and evil (that included, for instance, a knowledge of their nakedness) as well as the ability to increase in understanding of good and evil. That basic understanding of good and evil came through their conscience (literally meaning “inner knowledge”), which instructs every man sufficiently to know good from evil:
4 ...And the way is prepared from the fall of man, and salvation is free.
5 And men are instructed sufficiently that they know good from evil. And the law is given unto men.... (2 Nephi 2)
And the Spirit giveth light to every man that cometh into the world; and the Spirit enlighteneth every man through the world, that hearkeneth to the voice of the Spirit. (D&C 84:46)
And that I am the true light that lighteth every man that cometh into the world; (D&C 93:2)
With this key of additional knowledge, mankind was given agency they didn’t have before. Being made aware of good and evil, they were both free to obey or disobey their conscience, obtaining the outcomes of their own choice:
Yea, and I know that good and evil have come before all men; he that knoweth not good from evil is blameless; but he that knoweth good and evil, to him it is given according to his desires, whether he desireth good or evil, life or death, joy or remorse of conscience. (Alma 29:5)
And the Messiah cometh in the fulness of time, that he may redeem the children of men from the fall. And because that they are redeemed from the fall they have become free forever, knowing good from evil; to act for themselves and not to be acted upon, save it be by the punishment of the law at the great and last day, according to the commandments which God hath given. (2 Nephi 2:26)
And Eve, his wife, heard all these things and was glad, saying: Were it not for our transgression we never...should have known good and evil...and the eternal life which God giveth unto all the obedient. (Moses 5:11)