Bandwidth or how much data can we send out at a time is a primary considerations especially when many clients are browsing. Speed - The closer you are physically to a mirror, the faster your connection can be to that mirror. Reliability - Servers are pretty reliable, but sometimes a server may break down and not be reachable. If that's your only site, nobody can get to the information you want to show. If you have a bunch of mirror servers, a failure in one or two of them does not become a major problem.
Clients just switch to another mirror and keep on surfing.
Having all these mirror sites presents a challenge in keeping all of them up to date with the latest revision of your web pages. Every time a page is changed, an update needs to be sent to all the mirrors or the sites will appear differently. Sometimes this takes a lot of time. We have multiple mirrors with complete copies of the site. We also have special camera image servers which send copies of our live webcast images to each mirror as well.