If your on a fast enough connection, usually they will use you to get files off a site. So basicially you become a relay, allowing their slow connection to eventually get the files without fear of being booted off the main server. FTP easially allows redirection to another site, it was a common trick when many sysadmins were on dialup. They would FTP to a site then direct the download to their office computer.

Another possibility is they intend to turn your server into a main site.