It's a contributor license agreement. "Contributor Licence Agreements (CLAs) can be used to enable vendors to easily pursue legal resolution in the case of copyright disputes"[1].
Licensing/copyright can be a messy thing - especially with multiple authors involved. I assume the legal clarity it provides is seen as worthy of the extra hurdle to some of the more risky and/or cautious projects. Plus, depending on the agreement, it allows more freedom for change of license terms without getting all existing contributors to agree - the agreements I've signed have often contained a clause declaring implicit permission (or waiving rights completely).
We've found that it's a low hurdle for contributors-- it's a common attribute of most open source projects and helps avoid headaches of ownership later on down the road.
Essentially, no project wants the question of who owns contributed code to come up. The SCO-Linux controversy would have been avoided if a CLA had been in place, and since then they've become standard practice.
Licensing/copyright can be a messy thing - especially with multiple authors involved. I assume the legal clarity it provides is seen as worthy of the extra hurdle to some of the more risky and/or cautious projects. Plus, depending on the agreement, it allows more freedom for change of license terms without getting all existing contributors to agree - the agreements I've signed have often contained a clause declaring implicit permission (or waiving rights completely).
[1]http://en.wikipedia.org/wiki/Contributor_License_Agreement