What if you need something very lean for a specific purpose that React would do well, but it is just too big to justify it. That's ironically an edge case where React fails to deliver and this project fits neatly.
In my case, I wanted to take advantage of React-style components, but needed to use them in a massive (MM users) 10-year old website built on jQuery and Backbone, backed by PHP. Since there is no room for overhead, Preact fits very nicely.