Anywhere with a table. Use a couple lengths and layers of toilet paper laid out on a surface, aim head angle downwards so you don't miss the splash zone, clean up is easy, flush.

As for why, I don't like ruining an orgasm by stopping the wank too soon or by focusing on not making a mess, nor do I want to be jerking off in a bathroom. That's much less pleasant than a reclining armchair in a room I don't shit in.

southsamurai's method is good too so as long as you don't live in a regularly humid climate possibly, with not much evaporation going on? It's dry AF here so I wouldn't know.