I actually find some of them to be quite cute (the animal ones) but there's not much of a point to their existence when they poof the very next day. Is selling them the only real thing you can do with 'em?

I think the 'point' is being obnoxious. Like, changing the stereo station or turning it on/off, teleporting over your lot and being in the way. At least, that's the only point I've seen in them.

Actually they do have some functionality. See This Page Sims 3 Gnomes which details their functions up to EP 8.

This Page Mysterious Mr Gnome has detail up to IP.

Re poof the very next day: Do you have NRaas Overwatch installed? There is a setting there for cleaning up Gnomes nightly. I believe the default is True. I've got mine set to false and have a yard full of gnomes on one of my lots and they really are amusing ... well, in MHO anyway.

They're worth a lot of money. You sell them!

I don't have overwatch, but I do have Master Controller.

Ah, maybe this is actually what I've been witnessing:

Maybe the gnomes haven't actually been poofing but just teleporting somewhere else, where I haven't found them yet.

Where you do not have Overwatch, that is probably what has been happening. Just having one or two makes it harder to catch them moving about. On that one lot I mentioned earlier with all the Gnomes every so often I catch them moving around only to find them on another part of the lot or in the house somewhere.

I don't want to push anything on you but have you considered installing Overwatch? That mod keeps your game healthy. That mod and Error Trap are really must haves. Overwatch Functions Are Here and Error Trap Detail Is Here.

Just to clarify, if I may. There are two gnome functions in Overwatch.

One of them resets "stuck" (non-responsive) gnomes overnight. This one is on (true) by default.
One of them deletes all gnomes from the world entirely overnight. This one is off (false) by default and is intended for players who just really don't want them in their game.

The gnomes do occasionally teleport just off the edge of the lot, although master controller can be used to put them into the family inventory so you can put them back on the lot someplace.

The gnomes can be kind of cute with some of the situations you find them in, (although, if you have generations it can get both sad and irritating when your gnomes end up dying of old age long before your sims do, even when you've tried to leave ambrosia out for them. )