A crutch—also called a filter or a tip—is one of the easiest improvements to make to the standard joint. It’s effectively a mouthpiece, and it serves a number of purposes:
Keeps the end of your joint open (even when sharing with wet-lipped friends)
Adds support to a joint, keeping it straight
Blocks bits of plant matter from getting in your mouth
Ensures your lips and fingers don’t burn as you puff down to the roach
Crutches eliminate roaches and roach clips—you can smoke all the weed in a joint down to the crutch without burning yourself and then discard the crutch without losing any weed.
Blunts typically don’t need filters—blunt wraps are stiffer and the end can act like a crutch—but you can add one if you prefer. The only difference is that it needs to be bigger for a blunt.