Benjamin's answer was pretty clear but I'd like to emphasize a single point. CLA's protect companies using or contributing to projects. Companies, particularly large ones, are terrified of being caught off guard by legal issues. I've had corporate lawyers tell people I know that even in slam dunk cases where the law is on your side, you can only be about 60% sure that you'll win the case.
Consider the situation of a company, particularly a very large one with lots of revenue, builds a product around an open source tool. They gain customers, build out infrastructure and hire employees. If all of a sudden a contributor to the original tool sues them for patent or copyright infringment, the company is in a lot of legal hot water. If they stop providing their product, they've wasted money on infrastructure and employees they don't need and more importantly, they're pissing off customers. On the other hand, if they stop providing their project, their liability ends while the case in figured out.
Considering the current legal software landscape, CLA's are the best way to make companies feel comfortable using and contributing to open source software. It's not ideal (it'd be nice to never have to deal with any of this legal stuff) but right now it's one of the few ways of making these contributions pos