In a roundabout sort of way, it gives you the opportunity to see what someone else might do with a property if it was in their hands.
Don't have the rights to Batman? Go write Catman, Nightshadow, or Moon Knight instead. He can't fight the Joker, but he's more or less still Batman under a different name. You know you'd have fun writing a Batman story even if he had to fight a mad scientist instead of a laughing clown.
This character is Bruce Lee if he survived the apocalypse. This character is Bruce Lee if he lived in ancient China where people flew through the air, spat fireballs, and teleported short distances. This character is Bruce Lee if Bruce was somehow second best behind your character. (Blasphemer.)
Hey, look. It's that popular wrestler who works for the other company. Oh, wait. No. It's just his clone. Well, if you've ever wonder what a match between X and Y would look like, just pretend Z is actually Y. The match itself will function pretty much the same way as if it had been the other guy.
