The frame of a pack is supposed to function sort of like a recurve bow.
Think of pulling the load lifters on the top of shoulder straps as if you are pulling a bow string.
This tensioning is what transfers the weight of a pack from the shoulders down to the hips.
We want the frame to be stiff enough to resist curving under tension, but not overly heavy and cumbersome for the intended load.
Keeping with the bow analogy; a 50lb pull weight is preferred over a 30lb pull weight