Because it often forces the comic readers of (in this case) The Green Lantern and The Green Arrow to buy one comic and therefore vastly increase the profits of the company that made the comic.
It adds variety too, think about it, seeing the similar panels again and again can get kind of boring. *Superman uproots the road of the town and defeats Black Hole-O, the 17th person this week to attempt killing all humanity* It gets awfully dry, awfully fast, so having the skills and the characters mixed up, not fighting the usual enemies and not doing the usual things makes for a really good story without adding any story in the first place. So when you do add story, even if it sucks, no one will care because they are still thinking "Oh my god, that was so epic!"
Lets say you have Superman and Batman team up, as someone said before me "Utterly pointless" this is true but it looks a lot better when Superman comes down a punches one enemy in the face, while the exact same time Batman has gobbed in another enemy's eye, or whatever batman does. It adds variety and makes the comics slightly more bearable to people who try to kid themselves that superman isn't doing the exact same crap as last week.
