The only true reason is looks.

It depends on the traction were talking about. If its for drag racing its worse, the less sidewall you have the less traction on launch. Also width plays a bigger roll then Height. You have a 16" wheel thats 25" tall and you can have a 19" wheel thats 25" tall so size of rim really isn't a factor. The sidewall might shrink which would help in handling but going bigger then the front won't make a differnce.

MY 2 cents:

the Drag look has always been bigger in the back smaller in the front so you have this rake look, I think the import scene has adapted this idea and instead of changing tire size there changing rim size.

I disagree with the rolling better. Again Diamter plays ZERO roll in this. Width plays a roll, you can get any width with any diameter.

there is no performance in running 1 size bigger then the other. Changing widths is different story.