Why is a CLA necessary?

Thanks for this awesome project.

At the beginning of the README.md it is stated:

> IMPORTANT please read: This project for now does not accept pull requests, but feel free to open issues and discuss about bugs and features. PRs will be accepted later when a minimum level of stability is reached and I've the CLAs ready.

At the moment, this project is licensed with GPL v3.
Would you mind to shed more light into it on why a CLA is necessary here? No blaming or anything, I just want to understand it. Thank you. And enjoy your Sunday!