MH stores each email as a plain text file, each folder as a directory. It uses the unix filesystem as its database. It's very quick and has tools to re-order a folder quickly.
In addition, MH has tools to convert mail formats. It was designed in the days of low cpu power and small disks. It also lent itself well to being wrapped by other tools like xmh, exmh and mh-e so you don't have to learn the raw MH commands.
Yes, IMAP is cool, but don't discount MH. Plus the O'Reilly MH book is free as a PDF.
Oh, some IMAP servers and mail clients use MH format or something derived from it.