In our business (publication production) we need to make a lot of changes on the fly with story placement, widths and number of columns. Split/Span enables us to set an individual story (with headlines, bylines, subheads, etc. as a single object instead of two or three that can be moved and reformatted as needed. Yes, we could use a separate text box for headlines, of course, but then instead of addressing a story as a single object (as we do today in ID) we have to address each element independently. Just a bunch of extra steps and time on deadline when time is precious.
Until/unless Split/Span an option in Affinity's Publisher, it's just not a viable option for us.