Why write a business plan? Simple - the business plan is the blueprint for your business. You wouldn't walk over to an empty lot and just start nailing boards together if you wanted to build a house. Starting a business without a business plan is just as risky.

Taking the time to work through the process of writing a business plan will make for a smoother startup period and fewer unforeseen problems as your business becomes established.

Most new businesses need both startup and operating capital to get off the ground and without a well-developed business plan there is no chance of getting debt financing from established financial institutions such as banks or equity financing from angel investors (unless they happen to be family members or close friends).

Established businesses often need money, too, to do things such as buy new equipment or property, or because of market downturns. 

Having an up-to-date business plan gives you a much better chance of getting the money you need to keep operating or to expand.