I know an old plumber that did this if a toilet rocked or wobbled.
Code says to caulk around the bowl. I guess this is to stop sewer gasses from getting into the room. I personally think that's a bad idea because it just hides a leaking wax ring until the leak damages the floor. If you don't caulk, you'll see the leak as soon as it starts.
I've seen toilets set in thinset or tile grout on a tile floor instead of caulk. The toilet is then permanently part of the floor. Removal usually means breaking the toilet.
But leveling a toilet in plaster of paris has none of these problems. You can just put a dab in the four corners to stop wobble. It sets faster than grout or thinset. It's hard enough to do the job and the toilet isn't permanently stuck to the floor.