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

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

Boundary reinforcement issues can be addressed by taking them into account in the design stage. So can coloration due to wrap-around reflection energy (by using directional speakers, like the aforementioned Klipsch). The remaining issue is soundstage depth.

One technique which results in good soundstage depth is flush-mounting the speakers. This is often done by high-end recording studios. Precision to 1/8" or better is required, but the result is zero early reflections off that wall, so no corresponding cues are superimposed atop the soundstage on the recording. Note that most "in-wall" speakers have reflective and/or diffractive features which betray the presence of the wall.

Another is to design the enclosure to effectively blend into the wall, so that acoustically it’s like a "bump" on the wall. The midrange/tweeter module of the (sadly discontinued) Snell Type A fits this description, and its downward-firing woofer was close enough to the wall and floor to interact benignly with both of them.

There is another technique for getting soundstage depth with against-the-wall placement which I use in some of my designs. They work better with some spacing, but still imo better than most without. If anyone is curious I’ll describe.

Duke

Surprised there's been no mention of Larsen, one of the very few specifically (re)designed their models for operation right next to walls.

Audio Note also comes to mind as a speaker that favors close to wall/corner positioning.

90% of pics of systems in rooms that I see, I want to pull the speakers further out into the room, often vetoed because of WAF.