To answer your question, most people don't have just one device. Do you have only one device? You must have at least a desktop computer and a smartphone? What if you want to have something stored in your computer when you are not at home?
Music for example. If I don't want to pay Spotify or whatever, and I want to listen to my music on my phone at work and on my computer at home. Other than making two full copies of the entire music library, I think I have to store them on a 3rd location then share it to my two devices.
If I don't listen to music at home, then you're right, there's no reason to self host anything. I can just store all songs on my phone.
Seriously, don't buy the game at launch. Wait till the GOTY edition because many features that we had in the past will be packaged as expansions.