@Juan Ramon Zarco people hire a coach to help them perform their best and utilize the expertise they have at peak levels. It's different from hiring a consultant which in generally done to bring in expertise that is lacking. Although I generally do some of both, as many do.
In the best case a coach is only accountable to YOU not the company, or the share holders (to which the board of directors has fiduciary responsibility) though there are cases of HR people being coaches where that accountability is somewhat muddled.
In the case of business owners, who may also be the sole shareholders and directors of a closely held company a coach also serves as a sounding board who has no agenda other than success as you define it. In the case of an executive, a coach maybe someone you can discuss career change possibilities or work/life balance, company politics and other issues you may not want to bring up with your boss or your board - at least till you've made a firm decision.
In my experience, yes coaches do need some familiarity with what the client is experiencing, but often not being an expert in their field allows them to come at things from a new angle often with enlightening results.