Not really a good comparison. Quads aren't used nearly as much in squats as pecs are in bench.
Flyes really work the outer chest, near the shoulder tie-ins. That is where most of the stress is at the bottom of the bench movement, so naturally, doing heavy flyes should help with heavy bench. Basically the same way that heavy tricep work helps with the upper part of the bench movement. As said above, it all depends on your specific weak points. If you haven't been doing any pec work, then it may well be a weak point for you.