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

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".

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

klipsch and others do this as their approach, but it is like everything a tradeoff. If the speaker is designed for this boundary reinforcement then it will probably sound better used this way. It does affect more than bass response alone however. Placement close to a wall means early reflections that must be controlled lest they interfere with imaging. Search around see how many comments you find raving about the great holographic imaging of anyone's speakers that are right next to a wall. Whether designed for that or not they just aren't there. So that is one tradeoff they make. That's why there's so many different speakers. Maybe you like that tradeoff more than others. Then great, that's why they did it that way. Or maybe you don't. All that means is they didn't build it for you.

Makes sense that room constraints or manufacturer recommendations would be common reasons. 

I recall the bass on my klipsch forte iiis was insane against the wall, but I ended up pulling them out for soundstage benefits.

     Millercarbon made some good points.  I've always found that moving the speakers out from the front wall behind them resulted in improved imaging, especially perceived depth.   However, I have very limited experience with speakers designed to be placed against the wall in general.

Tim