Some celebrities do it for the timing of the publicity when they have a project coming out, like they would for any kind of television appearance. Some do it to remind the public of who they are. Seal did this season because his kids loved the first season and he thought it would be cool to do.
A few contestants have been open to how their confidence in their abilities were low and they needed to be reminded without the baggage of being a public figure whose history is well known.
I don't care that it's probably fixed in some way, mainly in the early weeks. It's fluffy and light, and it's nice to watch a show that you don't have to pay attention to plot and character. You get to turn that part of your brain off a bit, and we need that in our television diet.
Victor Oladipo in a Thingamajig costume is singing a heartfelt Kacey Musgraves ballad! What's not to love?