Whenever people ask why a person uses Linux, it's always the same key points:
- free and open-source,
- has better security,
- more customizable, and
- a bunch of technical stuff I don't understand
And whenever someone asked me why I use Linux, I would repeat those keypoints
But after using Linux for a couple of years, I can't exactly say any of that has any real benefit to me. I don't really customize my desktop, security has never been a big focus, and being free and open-source does give peace of mind but it's not exactly changing the way I use my desktop directly. I appreciate all of it, and the options are nice, but it's not the main reason I use Linux.
For me, it's just nice to use (better than my experience with windows) and I think that's enough. The UI, for gnome specifically, just works for me. Having what is essentially an app store is great, system updates don't force themselves down my throat, and the community is just nice (most of the time).
There is a time and place for preaching the benefits of Linux, especially if they're asking for those benefits. But in my experience, I find that if a friend casually asks why I use Linux, a simple "I just like it" gets them more interested, and if they aren't interested, you don't come off as a bit annoying.