. If you lay your mast on the ground then stretch your sail out on the mast but not in the sail track, you may find that the shapes don't suit each other so you can adjust your spreader arm settings to suit.
Each catsail has a curve in their frontleech. So its not a straight line but there is a point where the curve is at its max. This is called the external camber of a sail. Hoisting that sail in a straight mast track will always cost you extra force because the frontleech doesn't follow the straight line of the mast. The greater this external camber is, the more trouble you will have with hoisting.
When you bent the mast with spreaders you will make the effect of the external camber again smaller. But don't forget that only this external camber will create the vortex of the sail when pulling the downhaul!
So only because of that your sail becomes the aerodynamic motor you want it to be.
P.S. external camber may never be negative; that's why sailmakers often cut to much camber in their sails. In fear of too much bend masts.