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 Backbon

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.