Valve appears to have quietly added a Steam feature that people have been asking about for a long time now: the ability to add a game to your library without having to download it. That may seem like a small thing (and it is, really), but it's something players have only been able to do in the past via workarounds that didn't make for the most convenient methods. It's a feature that's pretty much only useful whenever you're downloading free-to-play games, but it does seem to have a few uses outside of that, too.
This new Steam feature wasn't publicized in any sort of patch notes for Steam this week, so this stealthy feature was only noticed by people who shared news of it online after their discoveries. Twitter user RobotBrush, for example, screenshotted the option in the image below (which you may have to expand) that shows the usual "Play Game" button beside a new one that says "Add to Library." Fellow Twitter user Morwull contextualized this for those who might not have understood what was going on from the first tweet alone.
Steam has added "add to library" button without needing to start installation process. At long long last. https://t.co/rO5ywsJHLo— Alejandro LL. (@Morwull) August 13, 2022
As others pointed out in the replies there and in other threads where this feature was discussed, this Add to Library option was already available to a degree. There wasn't a dedicated button for this action, and to achieve a similar effect, players could end the download of a game at the first confirmation window that pops up to have the game added to your library but not downloaded at that time. That workaround has essentially been condensed now into the one button you see pictured above.
For paid games, this won't really be helpful seeing how you'd have to purchase the game anyway before adding it to your library. But considering the prevalence of free-to-play games nowadays with no end to that monetization method in sight, this feature will only be more useful as additional free-to-play games release. It'll also be useful when claiming free DLC for a game you already own.
One caveat to this is that it doesn't appear to be an option available on the Steam site at this time and is only found within the Steam app itself. It's also unclear if this Add to Library button is present on games that are temporarily free-to-play or free-to-own seeing how there aren't any games being promoted that way on Steam at this time, but those offers are never far apart from one another, so we'll see soon if the button does indeed work that way, too.