Valve stated, that they just can't gather enough internal testing data so they're giving everyone who owns the game a chance to test the beta version before it's pushed to production:
Users that own TF2 should see Team Fortress 2 Beta available in their Steam library. Users that choose to install and play the beta version of the game will give Valve the data they need for:
- Testing and tuning various class, item, and weapon changes
- Testing map changes and variants
- Running higher level, game-wide experiments (like doubling player health)
- Testing new technologies without the risk of breaking the game
The initial beta will primarily be testing changes to Natascha, the Level 5 heavy weapon. Other updates include changes to a couple of maps along side a few balance tweaks. ln the beta, all achievement based unlockables are available to all players, but Valve says don't get too attached because they could change or disappear at any time.
Along side the official beta servers setup by Valve, users can also setup their own servers if they would like. If you do choose to play on the beta servers, Valve wants to you to report problems to TF2 Public Beta Forum.