In my experience it's tricky. I've been burned a few times by the "we totally love it, we're behind you" thing, until you actually need money :)
I'd say a document like that needs to address the following.
Audience (Community the event will serve)
Competition (which other events are out there, maybe a count, maybe their primary location, or size, etc)
If you can figure it out, demographic if likely attendee (I don't know this until after my events or at least after tickets start selling, but you might be able to do a survey)
I think lastly, what's in it for the sponsor? Either your full sponsorship options list, or a description, likely size of attendee pool, email blasts, etc.
Off the top of my head that's what I'd put into something like that, if i was making one