A Pong recreation that uses CSS Grid for the rendering engine.
And many more.
Most of these start for the same reason: testing these new things to the limit.
And wondering if a particular use cases is supported.
And most times that's all that comes out of it.
But sometimes the silly demo isn't silly anymore and it exposes something real.
It exposes a bug, or it exposes a new need.
And it helps my team make the feature better.