Email attachments are fine for sending small files, however it's not possible to send large files such as high resolution images, databases, sound or movie files as the sizes exceed the limits set by most mail servers. Additionally large attachments slow down mail systems for both the individual user and the organisation.

The FTP protocol allows you to resume broken downloads or uploads, so in the event you have transmitted 95% of a large file and the connection breaks for any reason, you can simply resume where you left off. The email protocol has no error correction so it's not unusual for large files to arrive corrupted.

Many email service providers restrict their clients to attachments of 4 MBytes or less. So even though your email is not restricted, there is a good chance your client is! Now you have the convenience of a private channel that does not restrict the file size.