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At a guess (this is kinda far away from my own linguistics education), it serves some kind of pragmatic/discourse function. Basically, it means something along the lines of "I am now going to tell a story", which is probably useful information for interlocutors to know how to modulate turn-taking when interacting with the speaker.

I see this mostly online and strongly agree.

I don't know why it rubs me so wrongly. Perhaps because (IMO) it's totally unnecessary and adds nothing except perhaps an air of pretentiousness.

It has one good use case, IMO: when you want to mention someone on Twitter at the start of a tweet but without solely directing the tweet at them like a reply. For example: "So @otherperson built this cool thing..." Some people use a single "." at the start of a tweet to the same end but I think it looks hacky.

You're really going to hate Seamus Heaney's translation of Beowulf, then.

You're right. Translating OE 'Hwæt!' as "So." does make me grit my teeth.