To secure finance.No sensible lender or potential investor will part with their money without seeing one.
To improve the business. A properly conceived plan is a major management tool that can be used to secure fabulous results.
Most business plans are probably written to help secure funding but business improvement is without doubt a more powerful use. A properly conceived business plan will help secure the needed finance and will guide the management team to the riches they desire. It is also the case that business plans designed primarily to improve business performance, are more likely to secure finance.