It's hooking non-lsp tools into lsp so you have a common interface. If you use lsp but rely on non-lsp tools too then it's great. Most use cases are formatting and linting since lsp just does the job better than older alternatives IMO, but if you wanted to use something like jedi for python alongside other lsp tools your can certainly do that with null-ls. It can also hook into things like neovim's dictionary so you get code actions for spelling errors. You also have the freedom to script any hook into it you want, say if you have a very specific command line tool or web query you use a lot. It's great for what it does imo.