Water is pretty small. why does it need proteins (the aquaporins) to cross the plasma membrane?

First of all, most of the cell is water, and if water could just flow freely then the cells could become deformed or lose too much water or anything similar. Free passage of water could also ensure free passage of other things that might be there so the Aquaporins are needed to allow some water to pass, without any other elements. Water passes through in a single file, meaning it doesn't just pour all out.