In the early days of AnimeSuki (especially during Spring/Summer 2003), it was sometimes hard to keep the site up and running. For this reason the scripts were adapted so that AnimeSuki could be mirrored on several locations. This way, even if the main site goes down, you can still access AnimeSuki from a mirror.
While these days the main site has very little down time, there are still reasons to use a mirror site instead of the main site. For example, using a mirror that is close to you could mean faster page loading times. Or if you are tired of seeing ads on the site, you can choose a mirror that doesn't have ads. Another reason may be that all mirrors show the Recently Added torrent list in the timezone the server is configured in.