The recent proliferation of email allows users separated by great distance and disparate networks to exchange not just messages, but files as well. Most mailers were never intended to handle file transfers. Earlier mailers were incapable of handling binary files, files with long lines, or extremely large files. FTP is still the preferred method for moving large files, but not all sites are directly on the internet. Most current mailers have built in mechanisms to deal with file transfer issues.
Some mailers make bad guesses and all mail is handled in a store and forward manner.
This means any mail you send might pass through an older and more primitive mailer and get broken at that point. We have had many cases where the file was properly encoded at the sending site and then properly decoded at the receiving site yet was totally useless. Sending an Excel spreadsheet to someone who does not have Excel or something that can read Excel is a waste of time and network bandwidth. While these files are technically ascii, they are structured ascii and the structure must be preserved for the file to be usable. This means the file should be encoded.