The structural integrity field counters the gravity forces placed on the ship from what I gather. Its in place during warp drive in case warp travel is disengaged suddenly, although not needed at warp. This is because warp fields actively place the ship in a warp stream of subspace that floats the ship from the structural constraints of mass acceleration. The though occurs that a warp tunnel or slip-stream affect would require structural integrity fields for the ship to survive if collisions with the tunnel occurred. In the two differing situations: Warp Tunnel = preexisting tunnel in subspace Warp drive = subspace plow-like maneuver collapses into subspace afterward EDIT: To note, Inertia Dampers make the ship inhabitable for people while experiencing gravity forces. Structural integrity holds the ship together while accelerating, decelerating, maneuvering at high velocity of normal space or under normal space gravity affects and changes. EDIT 2: I just wanted to clarify, mass acceleration problems place the ship in limbo too far apart in sections of small distances... the stretching under forces of acceleration between distances of the hull would make the ship tare apart on top of the forces faced on the brunt. But in subspace the ship does not experience the stress. the warp tunnel in voyager was held open by some sort of spatial phenomena to the region of delta quadrant, but I don't know if it was naturally occurring or not. Warp tunnels are typically smaller and give ships extra time off the trip but not much to boast.