In the late 1980s and early 1990, FTP traffic from popular file archives accounted for a large part of the traffic on the Internet (and contributed to the Congestion Collapse issues that were observed before congestion control was added to TCP). People in remote parts of the Internet (e.g. Europe) started to set up "mirror servers" to mirror the contents of these popular archives, so that the content would be available to the local community without putting load on expensive international links.
See Apache Scaling for a case study on a modern mirror server.
– Main.SimonLeinen - 08 Dec 2010