I've used them as components of plant mulch now and then.

i saved up a big bucket full to plant some blueberries. soil here is pretty alkaline, so, yeah, good for most anything.
but acid lovers especially.

Also roses love banana peels.
I prefer to compost stuff like that first, tho.

It's a good idea to add them along with a regular fertilizer or compost addition as the coffee aids in the development of beneficial bacteria which complements the fertilizer application.