There are two RFCs which describe more or less the same file format:
While on a desktop system usually there is usually no need to extract or create such archives, for the HTTP and mail servers this format might be an essential part of the functionality. p7zip might come in handy if in addition to the supported archive formats it would support MIME archives and HTML form-based uploads.
In fact, currently there is a very limited number of available MIME creating/decoding packages (e.g. ripMIME, Hunny MIME++), each of them suffering from its own shortcomings. Having another good tool which supports MIME fully, with a clean interface such as 7z, both application and the library, might have a value for those who found itself facing this problem.