The concept of runlevels goes back to the SysV days of, runlevel for a purpose. Different tasks require various daemons to be running upon system boot. This is particularly useful in server environments, where we try to minimize as much overhead as possible in our servers. We are usually going dedicate a role to a server. In doing so, we cut down on the amount of applications that are required to be installed on a given server. For instance, a web server would usually have an application that serves up content to users and a database used for lookup(s).
Another typical use case is a print server. This is usually only used for managing print jobs. That being said, from the perspective of runlevels, we would usually scale down on the amount of services that are running inside a given server. For those coming from a Windows background, think about Safe Mode...