So you can't sue us later claiming that we don't have a right to distribute your work.

It provides similar protection to us against patent claims. It also allows us to relicense Phabricator later if we need to without getting the approval of everyone who has ever contributed.

For a general issue in this vein, see the SCO lawsuits. Although the SCO issue was complex and far-reaching, one of the issues at its core was that IBM had contributed code to an open source project (Linux) which SCO claimed it was not within its rights to contribute.

The Apache Foundation requires contributors sign a substantially identical agreement, and the practice is common among a number of the more prominent and better-organized open source organizations, including Facebook, Google, Go, jQuery, Python, etc. You can find a more complete list on the Wikpedia CLA article.

The value of individual contributions in this project is generally very small, while the cost of tracking down every contributor to get approval to relicense or fighting a legal battle is enormous. Individual contributions aren't worthwhile for us to accept if they come with strings attached.