Any casual viewer of the BBC programme, Dragons Den will be aware of how easy it is for weaknesses or gaps to be identified. Depending upon the purpose of the plan, this may, or may not, prove to be critical. It is often easier to recognise such weaknesses and gaps, and be prepared to deal with them, either by noting them in the plan itself, or having appropriate answers available should the need arise.
Besides, your friends may be 100% behind you in your new venture, but, in case you are hoping to involve others who have actual money to invest, you may need to be able to make a convincing case. Wouldn't it be nice to have anticipated possible questions and be ready with plausible answers? If you are risking your own money, that is perhaps even a stronger reason to do some indispensable planning.