The principal purpose for the runners on most sloops is to deal with "pumping". Pumping, if you've ever experienced it, can occur in certain wind and seaway conditions. Usually it involves stronger winds and lumpy sea states. When you feel a rig "pump", you'll know it :^))).
We've had runners in our foward sail locker for 14 years for our J/Boat. We haven't had to use them. That's probably because I've avoided having that much sail up in rough situations.