Like all those old furniture or door locks that I just assumed were lost causes, many locksmiths can actually make keys for them. Not usually a serious need for it but if you were selling an old piece of furniture you might do it. In my case we have a bedroom closet we'd like to lock some stuff away in when we go away and have someone come in to feed the animals.

The ability to make a key for locks which have no key is another, even if those locks don't have removable cores (I'm thinking of mostly padlocks here). That's a really cool thing called impressioning, where the mechanics of the lock (pins pressed down into the keyway by springs) are actually used against it to progressively turn a key blank into a working key.

Master keying is another great example (when multiple, different keys work in a lock and you can have one key that works in multiple locks, such as a small apartment building).

Also a lot of ancillary things around the lock but not the lock itself, for example installing deep screws or replacement anti-kick hinges (door devil or door armor). In theory those are DIY things but in practice you can spend a lot of time trying to get alignment right (especially with a deadbolt going smoothing in/out), but it's something a locksmith does all the time.