Multiple subs gives you more even bass response in your room. Where you might be able to dial in one sub to give good bass response to your main seat, for example, room acoustics might prevent it from being great across all seats. The idea behind multiple subs and multiple LF sources (incuding full range speakers) spread strategically throughout your listening room is that by exciting as many room modes as possible you can minimize the occurrence of standing waves and nulls.
There are a good deal of articles about why, and how to, use multiple subs, including here.