OCD and/or the ability to make multiple passes without razor burn.
I've found the first pass is as close as a cartridge razor. However, because it isn't as prone to causing razor burn, I can make 1-2 more passes, getting much closer shaves. If I want the closest shave, I'll do two passes with the grain, then a final pass perpendicular to the grain. If I did even two passes with a multiblade cartridge razor, I'd get razor burn.
The first pass is enough, but getting that "Baby butt smooth" shave is a novelty for me, so I frequently do the 2-3 pass thing even if it isn't necessary.
Chris