I mostly use it for two reasons- my modern game has a ton of shapes and I hate scrolling through them all to get to a specific (usually older) outfit for default shaped Sims, and both my modern and medieval games have LBB, SBB and BB clothes because I like the extra variety in my muscular dudes... even if I mostly just use LBB for them... and the difference between them is subtle when they're not right next to each other.

But even if I had just a few shapes, anything that saves outfit-scrolling time in my laggy CAS is nice. Also, it means the mermaids and satyrs and werewolves are out of the way when I don't need them. (I'm beginning to sound a bit like an advertisement for the BSOK here, sorry.)

...I'm tempted to work out how to pin clothes to expansions, though, if only to get things like Viking tunics and other clothes for my subneighnorhoods and stuff out of the way, too- especially since when I'm not meshing I'm usually neck-deep in sim creation for my main hood.