My husband went with a general "I need this for personal reasons" approach (maybe we used general financial purposes? I don't remember) and that worked fine. Granted, had we been in a situation where being up front about immigration was less of an issue he may have just said so, but as a US citizen married to a US citizen (ie no connection to Canada and no unstable US visa situation) in a conservative state, we wanted to keep it secret as long as possible. In your case, it's probably much less of an issue to be direct about why you need it if you want since you're leaving soon. I do know a few HR people and they were clear that these letters are common for lots of reasons and a good HR person shouldn't pry and should be discreet about your request for a letter like this. It's really up to you.
