Supervision has many benefits besides automatic restarts after crashes. Supervising programs provide a consistent way to start, monitor, and log long running programs. Nginx reinvents its own interface for some of this functionality (like daemonizing, log rotating/compression, conf reloading, etc.), but it's useful for all services to work under the same interface. This is especially true for monitoring nginx's status, where a supervisor like runit is much nicer than 'pgrep nginx' or 'ps aux | grep $(cat /where/nginx/dumps/its/pid)'.

Like with all software... it doesn't until it does. And when it does you wish you had an auto-restart system in place.

It doesn't even have to be nginx's fault. It could be that some other process started fork-bombing the system and OOM-killer decided that killing nginx is the way to resolve it, before trying the actual offender.