The purpose of a mirror is to reduce internet traffic and to make the site more available. If you choose a mirror that has a closer location to yourself, the speed might be higher, the connection more reliable. Also the mirror maybe has not such a high load that the main site. Mostly there are user limits set.
If you do a KDE update, it becomes easily 500 MB of data to download. It is in your own interest to get the data as fast and safe as possible