I have both because I'm terrible at making decisions. Also, I grew up on PCs, so it's familiar, but I like that the iMac syncs with all of my other Apple devices (among other things). I have an iMac desktop and a PC laptop. I have a Surface Book 2 and I love it. It was pricey but well worth the investment.
I have an Apple laptop because I grew up with Apple products. It's what's familiar to me. I'm a huge fan of the new touch bars on the MacBook Pros. It helps me save a lot of time when I'm editing things. I've been thinking about getting a PC desktop though, just to have one.
I use a PC simply because I've been using them for as long as I can remember and prefer not to switch to something different. Part of it is that I'd rather not have to learn how to work Mac if the only thing I'm going to do with it is play music.