Its not that much about the businessplan itself as about the fact "ah ok, so these guys have actually thought this through and are serious about it"
Of course you should still have a good 10-15 slide presentation about the most important stuff, because people will actually look at that ;)
Planning can help with:
- How many talented developers might we need in 3 years? If we want to grow our own, when do we need to start hiring interns?
- At what size do we need to hire a receptionist and start thinking about more space?
- What's our burn rate, and when do we expect to run out of money?
- What rate of customer acquisition will we need to grow this business? Are we doing the right things to get there? If we do succeed, what's that imply for buying hardware?
- What's success look like?
- What's failure look like?