sometimes its a convenience thing, meaning no need to make an appointment just walk in.  I have used AMPs, but I much prefer making an appointment for personal massage.

For me it's partly the Asian thing, and yes typically more services are on the table, so to speak. Rates are moderate if I don't go for FS. It's convenient to be able to walk in without an appointment. And sometimes the surprise factor adds a little extra--it sounds odd, but I kinda like walking in with no idea who I'm going to spend the next hour being naked with.

 
Once in a while these places get raided, but I feel safe enough. If I saw anything that looked like a police vehicle sitting nearby, I'd put off my visit for another day.