It might be a bit of a chicken and egg issue at the moment: a native app is needed to be a share target (e.g. by custom protocol, or mailto as in your example), people don't really use native apps for these things any more, therefore widespread use is rare. There are some uses I can think of - e.g. Spotify registers a custom protocol handler, as does Steam. But in general I would expect enabling websites to be receivers of shared data will enable some interesting new use cases.

As an example, I wanted to create a web extension to automatically add something to Anki via AnkiWeb. The share API seemed to be the natural way to do this, but with it unsupported on desktop, that approach was dead in the water.