I have 20# backing on my 9wt lines, 30# on my 10wts

When you have a heavy (thick) line out, being pulled around every which way, plus a lot of backing too, there's a lot of drag on the line.
The drag on that line, plus the pull of the rod adds up to more than you might think
That stress is on the backing, not the leader.
Granted,you don't go that far into your backing everyday, but we're talking a fish that you probably don't want to loose.