User message like a notice, query, channel text and so on. It's useful when you change your mIRC layout (from <nick> to [nick], for example). I think it also has something to do with mIRC highlighting system (switch -l), when you choose "message".
Not putting the -m is useful when it's, for example, some kind of listing, counting or whatever. Ex: echo -s Listing banned nicks..
Hope I don't get that trout
Zyzzyx.