In order for the frame to flex laterally, it has to flex from end to end on either side Rooster, by stiffening the chassis and preventing the end to end flex it removes lateral flex as well.
Likewise when you stick a prepared front end under one, and a heavier rear axle, that works into the equation as well, with the sides reinforced.
If the pan is that light you'll probably also want to reinforce the engine mount using a little heavier plate under the engine, especially if you're running a modified engine, and you'll want to add a heavier plate under the tranny tieing that into both side supports. (unless you run a transaxle of course).
Everything you weld in there stiffens the chassis a little bit! The sum total of which adds up to a lot.