FTP servers allow companies to make data available for download across the internet.  Often this is in the form of promotional material like brochures, images, music and movies that might otherwise be too large to send via email.   

Companies may also use an FTP server to allow customers and remote workers to upload large files to their servers.

Some companies use FTP servers to allow external consultants with secure but limited access to their company network.  A good example of FTP server usage, would be to allow an external developer with the ability to update the source code for a bespoke application that they have written on your behalf.