Incubators will tear apart your ass if you don't have a reasonable idea of how to make money. Only dumb angel investors will go forward on just enthusiasm, and if you get a dumb one, congrats to you. Everyone else at least wants to know there's a business plan on file. And then en lieu of reading that plan themselves, want you to give them the 10-cent overview.
Having a formal business plan helps you put together that overview. Instead of "uh, well, there's the mobile market, of course," you hit 'em with: "The mobile market is primed for such an application. And we'll get them hooked with the social network integration when they see their friends posting through the app." Few people pull sparkling wine out of their ass, the rest have to smash the grapes, ferment the juice, and then let it age.
The hardest part of any business plan is answering the question, "Why would someone pay for this?" When you can answer that question, the rest is numbers. And Jarie Bolander's answer contains a lot of links on the topic.
If you can't answer the question, you don't need a business plan yet. But if you can't answer the question, you have to wonder about what you're doing.