Andy,
I can't speak to a 'typical' scenario for IMAP, but I can tell you our rationale for wanting to use it.
We view subsystems like IMAP, FTP, SMTP, Webdav and SharePoint as ways for clients (real and code based) to interact with the repository. If a legacy system can't be re-written to make restful web service calls, maybe it can FTP to be able to access Alfresco content. Maybe there is an external process outside of your network that sends status updates via SMTP, and you use that for starting or poking an Alfresco workflow.
For us, IMAP is useful for where we have a customer that works out of Outlook most of the day. We are able to create a mount point in their outlook that behaves just like an Outlook folder, and they can manually (by moving) or automatically (by outlook rules) move an email into alfresco. A new email may initiate a new entry in a case management application, or may be a supporting artifact for an existing case. As far as the worker is concerned, they are moving stuff around in outlook folders, but you have all the ability of alfresco rules, workflows, and other hooks doing the magic behind the scenes.
Good Luck!
