Libgdx is free, as in beer for you. For us it means covering recurring and one-time costs, such as paying for the build server, buying new Android devices, buying a Mac to develop for iOS and compile natives for Mac OS X and so on. That means less beer for us, which is bad.
We pondered long and hard about this and finally decided to put up a donation button. It's for a good cause, we code harder when drunk.