So Steam is still somewhat behind in this matter; other launchers, like Blizzard and Origin, allows you to play the game in its MOST VANILLA state. Essential assets are downloaded, animations are downloaded, voices, etc., and most importantly, the components needed to be able to communicate with the servers. It's when you start pushing beyond those that the download needs to be more "fully optimized" and up-to-date to, again, properly communicate with the servers.

Take The Old Republic for instance. You've downloaded all of the necessary models, animations, sound files, etc. to the point where you can at least get through first 10 levels. After that, for the game to properly function, you need the latest updates - i.e., things that they've updated since launch.