The security repository is segregated for at least one security reason: having security updates in with the other regular package repositories would introduce significant delays (compared to non-mirrored special purpose repos) for people who use mirrors instead of the official repo. By putting security updates in a special repo, and stipulating not to mirror the security repository, people use the security repo directly and don't have to wait for updates to propagate to their local mirror.
As for the other two that you mentioned, I presume their purpose is determinable from their names. Backports is likely for packages from newer versions which have been backported to older OS versions, and I presume archive is simply that, an archive.
