1) To tie him to Clinton.
2) To pre-empt the same accusation against Bush, for such notorious whoppers as 'nobody pulled strings to get me into the Texas Air National Guard', and his 2+2=6 supply-side tax cuts/Social Security privatization.
If you know that you're vulnerable to an accusation, make it first, so that the accusation gets turned into a 'you're one also' sort of mutual mudslinging.
Barry