For light ships, even one tick of rebuild is crucial between survival or not. You don't think of it as much but I've been in a lot of situations where we were just about to get it back up and then a shot landed. One extra spanner hit was literally what mattered the most.
Again, in heavy disable situations where you've got mercs or arts constantly shooting out a main gun, a wrench just takes too long and it often won't repair enough to make it matter if you can't get a repair in. Unless your foe is sitting right in front of your gun, you won't be able to get a shot anyways. Puggers will do this but vet pilots won't hang around and wait for you to rebuild, they'll be moving to blind spots or points where there is less risk. During that time, you've got engines going out, armor going nuts. If a gunner is able to leave the gun after a preload rebuild, then head back and rebuild engines while engineers focus on more critical tasks, that helps. Even if the engine doesn't stay up for very long, just being able to move slightly in some cases can make a difference for a pilot.
Heck they can preload rebuilds on multiple items faster and all an engineer has to do is come by and whack it once. In heavy tank situations, every second counts.