To solve the problem with non-US character-encoding.
Web browsers of 90s were notoriously bad with client-side encoding. The only way to show content properly for all available clients was server-side recoding and different HTTP ports were used to serve content with different encoding - ISO, DOS, Windows and KOI. Each web-page had a set of links, usually in top-right corner, labeled as "ISO", "DOS", "Win" and "KOI" which transferred you to corresponding HTTP port.
