In my mind signing is identity.

if you sign a post you are saying the person who knows the secret to this public key posted this.

If we think about a repost or a retweet, signing it would by a way of endorsing it or proving that a certain person wanted to spread the info contained in the post.

When it comes to self referential data (like a will or a contract) signing makes a lot of sense and almost matches up with what seems to be the normie understanding of it.

With creative data (writing, thoughts, art) it probably isn't as useful. But then, the idea of needing to sign art has always been a little funny anyway.