Acceleration gates are mostly used to decide which classes of ships can enter a site, and control the point at which players enter those sites or move between site stages. Usually these are deadspaces.
I think it would be interesting if some sites did not allow warping out except via acceleration gates or their equivalent. Presumably, one could still slowboat out of a deadspace site if they were sufficiently patient.
If nothing else, it would give the NPCs a sporting chance to win sometimes.