As mentioned, the tooth bar is more of a hinder than a help when working loose dirt. Even digging, the sharp straight edge of the FEL bucket makes a better and faster cut.
But for many jobs, it's the cat's meow.
..pulling up fence posts, with a 6 ft length of chain to wrap around the post, you have good stout places to tie onto the bucket with the chain.
..lifting/carrying anything else with a chain wrap.
..combing branches out of tall grass, and general brush-pushing.
..popping burried concrete blocks or mobile home pylons out during demolition work.
..just about anything else in demolition work - the tooth bar lets you carry more and hook onto stuff more easily
..popping out small stumps (if you can get a good angle on it; not near as good as a backhoe, of course, but better than a shovel and chain saw).
..take a run at a stack of railroad ties with the tooth-bar installed, you can get two, maybe three, railroad ties onto the FEL for transport; without the toothbar you have to dismount and hump the tie onto the bucket, and you can only carry one at a time.