L + R + SUB setting

i know what this setting does, giving the main speakers a full range signal and then the sub a copy of the low frequency from the mains. but just curious as to why one would use this setting? is this setting for if you have small main speakers?

It's mainly a personal preference. If you set your crossover point near the point where the speakers roll off, the result is actually not too different from setting the speaker to small.