Probably the most common use case is SaaS providers that support custom domain name for whatever the software it is. For example, a site uptime monitoring servi

Probably the most common use case is SaaS providers that support custom domain name for whatever the software it is. For example, a site uptime monitoring service might offer a feature to host a status page on a custom (sub)domain of the customer. The SaaS now needs to programatically create virtual hosts on demand, issue HTTPS certificates, run routine updates, etc. An API and a web server with small segmented updates make this so much easier. Compare this to Apache, that has to wait to properly end existing connections before reloading, has a config file parsing overhead, and probably does not scale that well with several virtual hosts anyway. There are hardware/File System level limitations as well.