I like using LLM's for exploration of a new codebase, seeing what other options already exist for solving a solution I might not be aware of and planning a bit how to change stuff, since it sometimes sees thing I did not think of.

Another thing it's good at is writing tests - a lot of times I won't bother, but with AI I can do it cheaply. And it's very good at keeping documentation and a codebase consistent, believe it or not. If I change a part that is mentioned somewhere else and it has it in the context window, it will update both parts, whereas I might omit it.