The choice of the second person as a narrative voice is an unusual one which is always deliberate. Second person gives a text a very particular feel: the reader is necessarily the person being addressed by the text. In the case of "Girl," the effect of this is that the reader is now the recipient of the barrage of information and instruction that is delivered to the girl in question on a moment by moment basis. As the recipient of all this instruction, we can clearly sense how inexorable it feels: the long sentences, broken by semi-colons, add to this sense of continuous instruction and command without pause. The story is exhausting to read; we can only imagine, then, how exhausting this life must be for the girl herself to lead.
There are, of course, weaknesses and downsides to this type of narrative. The chief among these is that we have no idea how the girl feels about all this, other than what we can assume based on our own feelings. However, this may also be seen as a positive thing—we are forced to place ourselves in the girl's position and feel with a true immediacy, rather than being told how this feels. We understand the girl's life directly, rather than at a remove.