I'm a long time Strava user. The entirety of that time it's been on private mode.
I recently had a riding friend ask to add me and when he did, he laughed at my antisocial party of zero friends and zero followers. Now I have one.
Awhile back, Garmin was struggling with their elevation gain, using a watch it would tell me I'd climbed 8000' when I knew it wasn't over 5k. Once uploaded to Strava, and cross referenced to the many other people who've ridden the same paths, this elevation discrepancy would get fixed.
That was why I started with Strava, it allows a much more accurate post ride breakdown. As a free user on private, I don't think I can even access the KOM's and other prizes. I just like tracking my miles for the year and looking at my rides.
The other thing I really have grown fond of is is the cross device capabilities. I've got a watch I'll use sometimes, an Edge when I can find it, my phone, and a tablet hooked up to my smart trainer. All upload to Strava and I don't have to make any effort about it. I like that.
I should probably pay, I use it enough. I'm just not sure what else that would provide me.