I think an oversimplistic explanation is probably that wrt the prevention of severe symptoms, it's still better to have some pre-existing antibody memory queued up even if incomplete and shitty rather than being completely immunologically-naive in the face of a new/first time COVID exposure.

So the shitty and incomplete antibodies produced by the COVID vaccines aren't good enough to prevent viral seeding and replication in the throat and nose following new COVID exposure, but likely can eventually hamper viral replication enough at other sites of the body that it buys time for other slower defenses to engage and prevent more serious consequences than your usual runny nose and sore throat.

Early antibody production addressing the initial infection earlier may also prevent other immune systems from freaking out and overreacting, creating cytokine storms and bad outcomes from self-destruction.