(This idea applies a lot to World Hub airports, but there is general applicability to everyone.)
Let’s say I download a World Hub airport and I discover that a part of a long PaintedLine needs to be EDGE_SOLID_ORTHO and another part needs to be EDGE_LINE_SOLID and then it needs to switch back to EDGE_SOLID_ORTHO again. Ideally, I would break the existing line in certain places, but instead, what I have to do right now is to delete several vertices in the line and then create a second line with vertices identical to the ones I just deleted.
I hope that makes sense!
A second use case: Let’s say I edit an existing airport and discover that half of the PaintedLine needs to be deleted. One quick way to delete it would be to cut the line in half where you want to delete it, then delete the second line. Unfortunately, without that functionality, I just have to go in and delete all the extra vertices. This has happened to me a few times, including right now where Blackshark didn’t notice a taxiway and I have to create it and rework the existing PaintedLines.
Outside of the World Hub, maybe you’re less likely to encounter this if you’re doing an airport from scratch and are more likely to catch your mistakes before creating 17 vertices you need to delete. But even still, over time, airports change you and need to go in and fix things. So it could happen.