There are not really any uses for locking things, AFAIK. NPCs don't steal stuff lying out in the open, but if you really wanted to you could lock your house (if you take over or buy one) to keep people from wandering in.
While in Aleroth I had Goemoe's cow in my house for a while until it wandered out again. Why do they have the ability to open doors? Only use I can think of so far is to keep animals from leaving fenced areas and the like.
