The 3 most important purposes of a business plan are:
1) to create an effective strategy for growth
2) to determine your future financial needs,
3) to attract investors (including angel investors and VC funding) and lenders
Why are FINANCIALS so important? ( Additional )
Business plan financials are vital because they allow you to budget for existing or future expenses, as well as forecast your business's future finances. A strongly written finance section also helps you obtain necessary funding from investors, allowing you to grow your business.