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

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.