Per Gygax himself:

"Adding a d% roll to an 18 Strength roll is hardly complicated. It was done because Strength was the only stat that needed to be increased in steps by the d% mechanic so as to improve fighters to hit and damage chances. So that was used because I favor interesting play over any imagined elegance, that being quite unlikely in an RPG in my view. RPGs are games, not art, and I don't give myself airs." "Can you imaging the increases of stats going into the 20s needed in order to get the same result as 18/00? The human norm bell curve of 3-18 down the tubes in a jiffy. Not elegant at all, and simply foolish ;-)"

Disclaimer, his views don't reflect my own.