Language development isn't driven by general communicative utility. The child doesn't talk better and better just to get more cookies, or to get more TV, or to be allowed to play outside more often. A lot of the changes you see in children's development simply make their speech conform better to the grammar of the language they're acquiring. Here's an example. Take a verb like "to cut," "to hit," or "to put." Children go through a stage in which they make errors like "cutted," "hitted," and "putted." A child at that stage is simply making distinctions that we adults don't. If I say "On Wednesday I cut the grass," it could mean that I cut the grass every Wednesday or that I cut the grass last Wednesday, because in English the past tense and present tense of "cut" are identical. A child who says "cutted" can distinguish the two, even though in some sense he is making a grammatical error. Children outgrow that "error," and in doing so they make their language worse in terms of the ability to communicate thoughts.