I've never done it to avoid reading the book. But when I was a kid, my dad used to demand I read something and then also give me the SparkNotes and demand I read those afterward. Well, he did that a handful of times, not like daily. The first time I remember him doing it was when he was really insistent I read Notes from Underground. I did, but I didn't like doing it.
I'll read the whole novel.
For some books I'll look up notes for certain chapters just to see if what I've thought while reading matches up with what others think and maybe see themes I've missed.
It also helps to make sure you've understood what's going on, especially in classics from the 18th or 19th centuries where it can be confusing.
I bought a bunch of Cliff's Notes once, mostly of well-known novels. I guess they're usually used for scholastic purposes, but I just thought they'd be a valuable read. I don't remember how much of them I actually got around to reading, probably like 1 or part of 1. I have no idea whatever happened to them.