We’re going with the standard answer – THAT DEPENDS!  If you are looking for funding (loan, investment, alternative financing, etc.) then you need a plan and the more complete the better chances you have getting funding.  Here are some statistics of having a business plan when looking for money.
You should have a business plan if you are in business.  But, it only needs to be formal if you are getting funding because that’s what funding sources want to see.  To run your business day-to-day we advocate what we can High Impact Business Planning.  This is a 2 – 3 page plan and a simple 1 page budget.  You will look at something simple like this regularly.  A 25 page detailed narrative plan, not so much!