Well, if you've ever had to write a business plan, you'll agree the projections for a startup end up being complete BS, but they make investors feel safe looking at a 20+ page document with lots of charts and numbers.

That said, business plans can be useful and help the founders solidify their business model, predict costs, etc. If anything, it helps you get a vague idea of what to expect a few years down the road instead of just the next 60 days.