To find connected regions on a map. I could probably have been smarter about the initial generation and connected things as they were generated (but that has only just occurred to me. Worth a go, I suppose.)

I also wrote a floodfill a while ago! We have a mapping app, and our imagery provider returns half-filled tiles at the edge of their coverage. And annoyingly, they don't have any metadata within the tile itself that indicates the coverage. They just return black pixels for a part of the tile.

So I needed a way to detect that, and doing a floodfill from the edge of the image turned out to be the easiest way.