TIL about Network Error Logging[1], which is having browsers send you reports that your DNS isn't resolving, your load balancer is timing out (HTTP / TCP errors in general), or your TLS certificate is invalid. Basically all types of issues that don't end up in your own application server logs.
Report URI should lead with this (it's not even mentioned on the front page)! It's super useful for every site, I can't think why anyone wouldn't want this. CSP monitoring on the other hand, which seems to be their focus, is a much harder sell, a pain to setup and maintain, and a bit questionable value in my opinion.