Free and open is important but my biggest reasons for using RSS are:
1. I get to choose what I see. No one can hide content from me.
2. It doesn't encourage Platforms. I can just as easily watch content from PeerTube or Odysee as YouTube. I can subscribe to any random blog just as easy as Tumblr or Medium. No need to for every author to be on every site that someone might look at.
2. is a direct consequence of being a Free and open standard. In order to be portable, it is best if it's a Free and open standard, so that anyone who wants to can interoperate.
It's related but not all standards allow for this sort of decentralization. IRC is an open standard and it still adds friction to be a member of multiple networks.
I use Liferea on non mobile.