When you post on reddit, instagram etc. you're giving the company your valuable data. (And it often is valuable, you put your thoughts into it and all that). Consider if you take 15 minutes to create value content, you'd get paid $5 or sth if you were doing it commercially. Now all that value is donated to big tech companies. (that's the only reason why they have value at all.)
So what's the problem with that? The problem is that reddit and others can instantaneously put your years-long effort behind a paywall. I'm not joking. Look at these:
when i want to see content on reddit (for research) or instagram (for local community) they don't allow me to see the data that users have contributed. Reddit claims that it blocks VPNs for "security reasons", meanwhile i'm using VPNs in browser's incognito mode to not be tracked. Instagram wants you to sign in sothat they amass more users and can take leverage of the network effect even more.
I can understand restricting who can post to limit bot activity, but reading must always be possible. After all, it's our data. That's exactly what happens when you give your data to big companies to "communicate with other people": your data becomes a source of income for them, at our expense.
A few browser extensions can help with that further. UBlockOrigin, NoScripts, CanvasBlocker, Privacy Badger, Consent-O-Matic, etc. Maybe Decentraleyes but I don't fully understand that one yet.
Then there's DNS proxying and/or DoH, and a variety of other settings that can be tweaked for maximum privacy.
Any one of these things alone will leave holes, but putting them all together can significantly reduce your trackability and fingerprint. Of course it's a trade-off because some of these will break websites, but most allow granular exceptions so you can still use the sites you need.
In any case, a VPN is an important part of that, but the main purpose is to make it harder for your ISP to track you. Obscuring your IP from web hosts is just a bonus.