the image in the link @WillAdams attached expresses it well?
if you’re not making a (large enough) chip because the cutter is dull then you want to increase the chipload and get a full scoop of mass that can carry some of that heat away. otherwise your cutter is getting carpet burn on the material - just rubbing and adding heat. it’s still spinning along into the material but heat is no longer being carried away by a chip.
same reason why most people burn out drills prematurely - they want to make a hole through a material fast so they crank up the speed and have it burn up rubbing and not creating a chip. feeds and rigidity play factor in this but it’s the heat that kills the bit