The majority of Internet traffic is FTP, meaning file transfer protocol.
It allows users to download files that will give them more information about your company. For example a catalog of your products or a price list. This allows customers instant access to your information and also saves on printing and mailing costs. Your ftp service will also allow you to have a simple FTP address such as /ftp.yourcompany.com/ that will appear to your customers as if you have your own dedicated server.
Ftp also provides a mean to others to download shareware/freeware programs that you have created. If you are a software designer/programmer you want to provide the most accessibility to your program. With ftp service you can provide a location for the download of these programs.