The CLA formalizes that you intended to contribute the work and that any other people with an interest in it (your employer for example) is okay with it. Here's a worst-case scenario:
Imagine that some employee of a company writes some nifty code and submits a pull request to jQuery. We land that code and six months later the company finds out. They're shocked, shocked that the employee wrote that code on company time and just gave it to us, especially because it was based on one of their super-secret header files and patented algorithms. They insist that we remove it immediately and remove that version from all CDNs. So we do. And everyone using that code has a broken site.
Now of course, that could still happen with a CLA in place, but only if the contributor lies or there is a misunderstanding between employer and employee. We're not seeking ironclad protection here, only reasonable due diligence.
Imagine that some employee of a company writes some nifty code and submits a pull request to jQuery. We land that code and six months later the company finds out. They're shocked, shocked that the employee wrote that code on company time and just gave it to us, especially because it was based on one of their super-secret header files and patented algorithms. They insist that we remove it immediately and remove that version from all CDNs. So we do. And everyone using that code has a broken site.
Now of course, that could still happen with a CLA in place, but only if the contributor lies or there is a misunderstanding between employer and employee. We're not seeking ironclad protection here, only reasonable due diligence.