Explanation
In short-form, b-roll does more work than its name suggests. It is the main tool for controlling pace, and pace is what holds retention through the middle of a video where most drop-off happens. A cut to something new resets attention; a static frame for eight seconds surrenders it.
It is also the cheapest fix for a video that is nearly working. Very often the script is fine and the delivery is fine and the video still drags, and the actual problem is that nothing changed on screen for long enough that the viewer's thumb got involved.
Sourcing b-roll used to be the tedious part: either you filmed it, or you paid for stock that did not quite match. Generating it against the specific line it sits under is a meaningfully different workflow, because it can be exact rather than approximate.
The common mistake
Using b-roll that is merely thematically related. Footage that does not match the sentence it plays under reads as filler and costs attention rather than holding it.