Linux can be used for many different tasks. It can be a server (a file server, an application server, a database server, an Internet gateway, etc.), a workstation running desktop packages (like word processors, spreadsheets, graphics programs, etc.), or even an X-terminal (a graphics terminal displaying windows of programs running on an application server).
Askrunlevel allows you to pick the operation mode right at boot time. Generally, you will have a default operation mode and a timeout which allow your computer to boot and start normal operation without user intervention. This is especially useful for servers.