User metrics is a very common way of measuring a product success, be it a small startup or products like Yahoo Search, which I currently work on.
Most of the motivation for this new spec is expressed in the spec itself and other browser vendors are already onboard and have this Last Call Editor's draft spec on their stable version of the browser.
In our particular case, we would appreciate the `navigator.sendBeacon` API not only for the reasons stated there, but also we are very found of the following paragraph:
> "The User Agent SHOULD transmit data at the earliest available opportunity, but
> MAY prioritize the transmission of data lower compared to other network
> traffic. The User Agent SHOULD make a best effort attempt to eventually transmit
> the data."
Specially in areas with low coverage of cellular data access, this would mean a lot perceived performance improvements for the end-user, as we could use the new API to send all user metrics data, not only the clicks that happened before unloading the document, and Safari could potentially hold this requests until user has higher bandwidth available. Our search results would potentially load faster, the website the user is navigating to would load faster.