"Plans are useless; planning is indispensable," according to Dwight D. Eisenhower, then Commander of the Allied Forces in Europe during WWII. Now, you may be in total agreement with the first part of that statement, but you are really not convinced of the truth of the second part.
When writing a business plan you sometimes end up locking yourself away. You might have unique ideas which lead you to seek some isolation and secrecy, or if you are going to be a sole trader you may only have one person to consult namely yourself. But it is fantastic to try and get broader input on your business plan _ whether from a professional, or simply from friends and family whom you trust. I say that because of course you need to be careful with commercially sensitive ideas, as you do not want to pass your plan on to someone in the pub who then starts your idea before you across the road.