The CLA clarifies the responsibilities we as a company have, and what you as a contributor have. It gives us an undeniable record that the contribution was sent to us willingly, and that the contribution is given to the open source project unconditionally.
It helps us guarantee, to the extent possible, that the open source project stays open source, and the code in it is licensed properly. It is a protection against, as an example, people acting in ill-faith, by contributing something and later trying to get the contribution removed, causing harm to the open source project.
Traditionally a contributor would need to print the CLA, sign it, and physically e-mail it to us. This process takes a long time and is hampering the productiveness for the contributor, and us as open source project maintainers.
Inspired by others, we are collecting signed CLA's by accepting them as close to our open source projects as possible, here at GitHub.
We believe it is a lot more convenient, and stream-lined, for everyone involved and we hope you feel the same.