It's a bunch of factors. Here are my reasons for one:
1) Additional cook burner. I can cook on a PS - especially if I'm warming up a big pot of something or cooking pasta.
2) Dish water heats up in minutes - as opposed to probably 10x or longer on my regular cook stove
3) Space - since it sits on the ground it doesn't really take up additional space.
4) Simple - there is not much to the PS so I consider it to be a reliable backup in case something happens to our primary cook stove.
5) Fire starter - it works wonderful as a wood/charcoal fire starter as well as anything else you want to torch