Linux can be used for many different tasks. It can be a server (file server, application server, database server, Internet gateway, etc.) or a workstation running desktop packages (word processor, spreadsheet, graphics, etc.). It can even be an X terminal (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 allowing your computer to boot and start normal operation without user intervention. This is especially useful for servers.