That may be regional. Seamus Heaney says, "in Hiberno-English Scullion-speak [...] 'so' operates as an expression that obliterates all previous discourse and narrative, and at the same time functions as an exclamation calling for immediate attention."
Which sounds pretty weaponized, to be sure.
On the other hand, my (Canadian) experience of it, when used to introduce a topic shift, is that it serves as a warning, to make the transition less jarring, not more so.
Biting the hand that feeds IT
