The major motive for using message conversion is the need to convert character set of a message text. The reason for this is that character sets in internet messages (at least outside of the English speaking countries) is a mess. When the common characters in US-ASCII does not fully support the national alphabet, character set really matter when you receive a message text.
The second motive is encoding. The problem is similar to that of character sets. You normally do not support more than one or a few encoding standards on your local mail agent. Therefore it is a big gain in usability if you can convert messages to use the encodings your mail agent supports.
The third motive is message format. The most commonly used formats for Internet messages in Sweden is plain old style rfc822, Sun Mailtool, and MIME. Neither of those formats is suited to interprate any of the other ones.
ITS Uppsala university