When you sell any product you need good marketing materials to communicate all the salient points that will convince a buyer to look closely at your product (company stock) – those materials are needed to get a meeting with your buyer (investor) to present your case.
To sell stock in your company your marketing materials are the Executive Summary, elevator pitch, and PowerPoint presentation (a business plan will only normally be needed after you have attracted investor interest). Your goal is to get an investor interested enough to get that meeting.
It's worth spending time on these materials to get them right since normally you only get one shot. And make sure that they are oriented towards the kind of investor you are seeking (see Part II of this series).