"I'll know it when I see it" may work when looking at a restaurant menu, but rarely when looking to add exceptional talent to the team.
When a leader knows what the job functions are, they can create an Accountability Matrix for the position. This includes the three to five primary accountabilities, their relative priority, the percent of time expected to be associated with each accountability and the success factors which will determine up front, whether or not someone has been successful or not in the described position.
This planning allows the hiring manager to focus their interviews on the key components of the job. In the absence of this planning, they may be attracted to the person most likable or the best "salesperson" interviewing for the position - which may be fine if they seek a salesperson but it may lead them down the very wrong path if those traits are detrimental to the position.
Empowered hiring managers succeed by creating clarity around the position and a plan to select the individual who best fits that role.