OK Jon - it looks like your spidey sense is spot on!
Appended columns just means that two columns share a heading.
On its own, it probably isn’t especially useful for a large number of users. However, it was required to deliver the enhanced stream mode (introduced in v7.4.3), mainly for legibility reasons, column-width reasons and repetition reasons, which becomes quite significant with complex queries requiring double height headers. Enhanced stream mode is an important feature because it makes multi-stream comparisons simpler, without needing custom mode:
Although I don’t mention it in the video, the enhanced stream mode requires the appended columns to work. You could try it yourself: go into stream mode and then jump directly into custom mode. You’ll see the appended columns at work.
I hope that helps!