Why use a carrier if fighters share its propulsion?

If fighters can use the same propulsion technology, why have a carrier at all?

Because the carrier preserves formation, support, specialist capacity, and a safe base while smaller craft perform risky local missions.

Giving fighters the same propulsion technology does not remove navigation, support, or concentration problems. Tiny angular errors over interstellar or intergalactic distances can scatter independently traveling craft so widely that regrouping takes days or weeks. A carrier delivers a coherent task force and carries the specialist people, maintenance capacity, and analytical systems needed far from home. Once on scene, it can send expendable vehicles into uncertain reconnaissance or combat situations instead of risking the whole ship. Distance itself becomes the main defense for the expensive parent vessel.