I was often asked in interviews why a successful executive would need a coach. 

Only the racehorse needs a coach

When answering this question, I always draw on an example from sports: only the really good athletes have their own coaches; it is not worth investing in a coach for also-rans. An individual who is offered a coach should not feel there is amiss with him or her but instead view it as a token of appreciation from their company that is willing to invest time and resources towards their betterment. 
It is not the workhorse but the racehorse that needs a coach.

Toolful Coach: SPARKLE coaching model with 150 useful tools and case studies by Laura Komocsin (Author)