For one, "owning" means that there is no one, except you, who can singlehandedly delete or censor content that you published on the social media platform. This happens regularly e.g. in Turkey to silence opposition. Most recent notable case https://twitter.com/MahirZeynalov/status/780420919470751744

And that's the tip of the iceberg.

I recall quite a few of these solutions letting you post to both the decentralised service and the likes of Twitter at the same time. So your old audience would still see your content, and (theoretically) be gradually encouraged to move over.

I think in a better world/internet (need not be perfect), you would post your content (posts, tweets, whatever) on your site first. Or, maybe a decentralized site that you might not control alone, but is a separate community from the big silos (e.g. https://quitter.no)...And then, your content is accepted (pushed out) and able to be read by users who only live (or spend most of their time) on the big silos. In other words, it is possible to have both worlds that you noted.

This would allow freedom for indie authors to own their content (in the scenario where one of the big social network silos shuts down, or the big boys for some reason wish to censor an author's content, etc.) and post their content however they wish and from wherever they wish, but still allow them to retain audience (those users who are fine just living on the big social network silos)...This is not unlike email, where i can send a message to pretty much any legitimate email address regardless of the silo (I'm leaving out spam blocks out of discussion of course).