Mozilla does not want to support PWAs in Firefox, yet MDN has a PWA manifest and can be installed in Chromium.
(discuss.tchncs.de)
PWAs are useful on desktop if there's web apps you use a lot every day. For example, some people at my Workplace are in Google Docs a lot, so a Google Docs PWA would be useful. Separate taskbar/launcher icon, separate window in Alt-Tab, and at least in Chrome, Google Docs has some basic support working while offline.