The purpose of this beta currently remains a mystery, with the only official comment from RobTop Games teasing that you need to guess the password to access the beta.
In response to a question on the Geometry Dash Discord server asking if the Geometry Dash beta branch on Steam is for 2.2, RobTop replied with the following:
just input the key and you will see
This beta branch is protected by a key. In order to access it, you need to input the correct key. As of now, this key is unknown.
It's possible that RobTop will give access to professional beta testers, but given how RobTop has infamously been a lone wolf when it comes to 2.2 development, this branch likely only exists to test the game on Steam to make sure 2.2 works with Steam's systems.