I originally ordered Fallout 4 through Amazon to get a box, but when I saw their estimated 3 - 5 day delivery time.... even if they offered launch day delivery, I'd be skeptical (and it would cost almost as much as the game, in postage).... so I cancelled and went with a Steam purchase. My Fallout 3 runs without Steam but when I got the New Vegas GOTY edition through Steam, it requires me to run steam. A little irksome to have to launch Steam and THEN launch my Nexus Mod Manager which launches a 4GB Memory extender EXE which launches NVSE which then launches Fallout New Vegas....but, still, I notice no performance hit.
And I have to admit, being able to download all my STEAM games onto a new computer instead of finding the physical disks is preferable. I just shudder to think what would happen if STEAM / Valve for whatever reason ~stopped~. Then I have no physical backup.
I'm also not sure the game would run "through" Steam if I had no internet connection (for whatever reason). I know you can run games in Offline mode, but I never puzzled out whether the game would even START without at least handshaking with Steam. Having a single player game that I cannot play without going online is kind of irksome since, if I could not go online for whatever reason, it would be nice to be able to play a single player game that in and of itself, required no internet.