valve is test an option In the new Steam client beta that will force games to only download updates by default when you launch them.
Currently, Steam by default decides which game updates to download based on a few different things. As explained by Valve:
For games you've recently played, Steam will download the game update immediately after it's released. For games you haven't played in a while, Steam may wait a few days to bundle multiple updates together or choose to download updates while you're sleeping.
This new default option to wait to download updates gives players more control; As Valve explains, this means users can keep updating a big game until they actually want to play it, which could be a relief for those with bandwidth caps.
You'll find the new option in the beta Steam client settings under the Downloads section. “You can set the default: let Steam decide when to update the game (based on factors such as when you last played the game, bandwidth availability, etc.) or wait to update until the game launches ,” Valve says.
You can still set per-game download settings in the game's properties that override the global setting, but “details in those options have been cleaned up,” according to Valve. And with the beta, you can also manage per-game overrides in the overall Steam client download settings.