Be sure to read the fine print. The main point of the CLA for most corporate-backed projects is to allow re-licensing under different terms (read: proprietary). This does not require explicit copyright assignment.

On the other hand, the Free Software Foundation requires copyright assignment to give the FSF standing to defend the copyright - to enforce the GPL.

> It only provides the project with a license to use the contribution

Most projects simply specify that any contributions must be made under the same license as the existing code. The act of voluntary contribution is considered (by most) as sufficient to establish assent to this, although many projects take it a step further and require copyright statements in each file header. For projects that never plan to re-license, this works fine.

One reason for this is in case someone uses GNU code without following the GPL. With the FSF as copyright assignment, then the FSF can sue the organisation breaking the GPL. Otherwise the individual programmers would have to do it.