Who puts their speakers right up against the back wall and why?

those who don't have room to put em out in their rooms?

The farther woofers are from room surfaces the more uneven the frequency response in the bass region! Roy Allison of Acoustic Research (AR) discovered this decades ago. Now known as the "Allison Effect".

Allison and Snell. The most even bass response is from having woofers near the boundaries of two surfaces (wall and floor). This also applies to subwoofers.