Re: Reason for creating team.
Any menace more powerful than any single hero should be reason enough: a rampaging robot, invaders from subterranea, sea monster on the loose, weird science gone awry, a powerful wicked witch, etc.
You could always go the Defenders "non-team" route, where they only come together when there's a menace worthy of their combined powers and aren't a formal team with rules and memberships.
I try to avoid having them gathered together by another individual or agency (retired hero, UNTIL, etc.) as I find it works better when the PCs are self-motivated to take the initiative to form a team or to band together.
--Kap