malymin

I only saw the special interactions with mermaids/werewolves occur when I taught my fairy the "cure/apply ailment" perk. Once you can use it, you occasionally get unplayed mermaids asking you to hydrate them (filling the Hydration motive) or werewolves asking you to soothe them (emptying the Fury meter), but you can also use these interactions on werewolves and mermaids in your own household. So fairies make good room-mates, spouses, etc for werewolves and mermaids.
Using "soothe the beast" on a werewolf seems to count as a positive interaction for the purposes of the "warm force gain" passive, and thus builds up emotional force instead of draining it. Same is true of "water therapy" on mermaids.
Fairies also have a special power for vampires, but this is just... curing vampirism to turn them into a normal sim, so not helpful if you want to keep a vampire as a vampire.
Fairies' "nurture nature" ability is currently the only occult power that directly helps pet-sims (cats, dogs, horses), making them the best pet owner of any occult. Meanwhile, spellcasters can't cast Scruberoo on pet-sims, even though both person-sims and object-pets (livestock, etc) are valid targets.
Interacting with pets counts for the purposes of "warm force gain." Haven't tested livestock yet.
Gathering and using fairy dust builds up fairy experience, even if it's gathered and used in a way that's also available to non-fairies.

A few notes on TS4 fairies