Gene Roddenberry had a pretty good idea In the Beginning when he said the speeding Enterprise would need a deflector beam to sweep aside particles that lay ahead. Along with the specks of dust he had in mind, which are not a big deal between the stars, there are hydrogen atoms in outer space, about one per cubic meter. If you fly through these atoms fast enough, the effect is just as if you were standing still and getting blasted with hard radiation. It would be fatal after a while. So giving starships a forward-facing deflector dish was an even better idea than GR realized. It probably saved the lives of Star Trek's entire cast. But I think the writers, especially early on, may have conflated the deflector beam with the shields, which Scotty once called his screens. They even used the term "deflector screen" in "Mudd's Women," when it had to be the shields. [Mudd's ship would be pushed but not protected by our deflector beam.] The shields are for combat, blocking enemy transporters, slipping enemy tractor beams, and surviving collisions if any. You can also use them to cover Harry Mudd's ship in an asteroid belt, but it breaks your own ship (and it was worth it, it turns out). The deflector dish is for interstellar hydrogen and pushing asteroids out of a planet's way. The early confusion of terminology might lead us to think the two systems are inter-related. One thought that arises is: as long as you have the shields (which presumably put the ship in an egg-shaped bubble of safety), maybe you could run them all the time and you don't even need a deflector dish. It certainly seems like a belt-and-suspenders arrangement to have both systems. You can always use the tractor beam to manhandle asteroids; don't leave home without it.