As has been speculated, to fish for a denial. Thus, making the affair more than it really is.
If the White House denied the typewritten documents, then they sprung the handwritten documents, CBS and the Dems would have grounds for claiming that Bush was not only a slacker, but caught in a lie, as well.
Presumably, the handwritten documents would also reflect and correct the errors present in the Word docs -- such as GWB's Longmont address, the date of Col. Staudt's retirement, Army terminology, etc.
It's all too clever by half, I agree. But we're dealing with some devious and diseased minds...