I also completely understand someone who enjoys something like photography or filmmaking taking the time to document their experiences.
However, I doubt most people who post on social media fall into either of the above categories.
I'm a bit socially inept, so if I'm at a gathering and know (from Instagram) someone outside my immediate circle that I'm talking to has recently holidayed with their kids or got a pet or whatever, it's a big help prompting conversation.
For me, appearances and experiences are part and parcel. Let's say I like going on adventures and that I like people to think of me as adventurous. And so being adventurous to demonstrate that I'm adventurous is self-serving. Gets me off the couch in those moments I need a little push.
I create content because it preserves memories and/or more than pays for the experience, enabling more trips. I travel, take photos and videos, and then sell media afterwards. Just like other people watch TV or movies, I enjoy editing photos or putting little movies together from footage I've shot while having a glass of wine.
Last year, on a particularly large trip, I took a lot of video to document it: https://shubbo.com/us/ - now I wish I'd taken more. I love looking back over it and reminiscing, as does my family.
Sometimes I feel like you can create content or consume it. I enjoy both.