
That can be useful if you leave Steam running all day, as when you get home from work you will find it has downloaded patches for you in the background. Note that if the game is left "installed" on Steam it will still want to auto-update. You can bypass Steam and run the Frontier launcher standalone, which will let you choose to install only Horizons. Nowadays there is no 32-bit client and I believe that the base game and Horizons install into the same directory (don't quote me on that) but it wouldn't surprise me if Steam wanted to install everything from a clean start. OP is right that Steam used to want to download all three, which was a waste of time, bandwidth and disk space if you only wanted to play Horizons.

Elite Dangerous: Horizons (64-bit), Elite: Dangerous (32-bit) and Elite: Dangerous (64-bit) are separate SKUs and used to be installed into separate directories.
