The plan is useless; it's the planning that's important
-- Dwight D. Eisenhower
I interpret this to mean that the planning process forces you to understand the problems you face, and that understanding is what lets you adapt to changes in circumstances.
Sure you may not need detailed 3 year financial projections or spreadsheets coming out your ears, but if you don't have at least a basic plan then you run the risk of having to handle issues you should have seen coming if only you had a basic plan. Writing a plan is as much for the Founder as it is for any investors and only fairly experienced businessfolk can really get away with skipping it.