I find it useful for filtering and caching. Things like redirecting traffic to /.well-known/acme-challenge/ to your certificate management host, providing an endpoint for machine status or filtering requests to dot-files. Or telling Nginx to cache responses and allow it to serve stale content when the backend server returns 4xx/5xx status codes during deploys or high load. Handling things like client certificate authentication in Nginx instead of doing it in every backend application is another thing I've found useful.