The business plan is for you to prove to yourself that you have looked at all the costs and competition, worked out your suppliers , selling price and associated costs. That means in real terms you have thought of the majority of potential problems and costings and can make a profit at the end of the year
The next part is to make a simple cashflow forecast based upon the plan (available from all banks for free) to prove to yourself that you wont run out of money after a few months
Your investor would like to see your plan
Is a business plan a requirement, No you can wing it especially if your in the same trade and know all the answers (leave your company and start a competition), but even then Cashflow can come and bit you