I think it’s a matter of how much you insist on software freedom. On a scale going from “don’t care” to “fanatic”, something like this:
Don’t care about free software: Windows/MacOS users
Appreciate free software but don’t mind some proprietary parts: common GNU/Linux distros like Ubuntu and Fedora
Want to use only free software but accept that proprietary parts may in practice be necessary in some cases: Debian GNU/Linux
Insisting on using only free software and won’t even consider anything else: PureOS/Trisquel/other FSF-endorsed GNU/Linux distros
Two reasons I can think of: (1) you want to be really really sure that you are using only free software, and (2) you want to push the world in that direction, help creating more incentives for making things work with free software.