apt-mirror is a handy tool for creating a replica (or ‘mirror’ if you will) of a Debian archive. This can be useful in a number of circumstances if you want to maintain your own archive. Having your own up to date archive means you have fast access to every package in the distro very quickly.
Maybe you need it for installing a Debian-based distro onto lots of machines on a LAN. For example a system builder who installs Linux on many new computers and wants to improve the speed at which the systems are updated after a clean install. There are of course other tools that can achieve this including apt-catcher, apt-proxy and squid. These are in fact arguably better than the apt-mirror approach. But this is more fun 
