During a new release, servers are very busy and strained, and it can be time consuming to manually find a usable mirror. If you want higher reliability and automatic error correction* with your downloads, or can't use BitTorrent where you are, there's Met
If a server goes down during a download, download programs can automatically switch to another mirror. Or segments can be downloaded from different places at the same time, automatically, which can make downloads much faster.
G. Bracha, and D. Ungar. Proceedings of the 19th annual ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications, ACM, (October 2004)