Half-Life: Alyx is scheduled to be released in March 2020 and is Valve’s first title geared entirely to VR play on the PC. To promote the upcoming game, you can now play all of the previous games in the series for free for a limited time. This is essentially a two-month pass to the games, and when it’s over, you will not be able to continue playing without purchasing the games. The games are excellent and if you have not played them before, I highly recommend you do so while they are free.
Source: https://www.bleepingcomputer.com/news/gaming/the-half-life-series-is-free-to-play-until-march-2020/