You would not want to disservice a client by potentially purchasing at too high a price, or marketing at too low a price. Marketing at too high a price might also disservice all parties - as finding a buyer becomes slimmer and time costly.
Often business owners are in disbelief that their business is not worth what they had imagined (as much as a Facebook or Apple for example). Obtaining an independent valuation helps to ascertain and clarify a justified fair market value. The buyer is only concerned with what profits the business can generate. It can be a tricky situation, but an independent valuation can help an owner to look at their business objectively and to settle on an acceptable price sooner.
For a potential buyer - an independent valuation helps give further confidence and conviction in the company's valuation, negotiations, and eventual purchase price.