Not all CLAs are designed to let the project relicense itself. In particular, some CLAs are there to make the contributors promise that they are not submitting

Not all CLAs are designed to let the project relicense itself. In particular, some CLAs are there to make the contributors promise that they are not submitting patent-encumbered code which they intend to later assert some rights on. Those CLAs seem like they are fixing up a fundamental unclearness in how patents are licensed in open source software.