As mentionned you can use it in both, i am on Steam because i want to centralised all my games in one place, the tricky part is, how to load the game in VR using Steam, here is the 3 links i bookmaked back then that might help, agian it is being a while, so not sure which one is the most useful, i am just sharing with you, but bascally you do not launch using Steam VR.