In a game, the goal is to score points. There is generally a scorekeeper who keeps track of how many points have been scored. The scorekeeper does not participate in the game, but he or she keeps track of how well the participants doing in achieving their goal.
In business, the goal is to make money. An accountant keeps track of how much money has been spent and how much has been made. The accountant does not typically participate in spending the money or running the business, but he or she keeps track of how well the business is doing in terms of achieving its goals. Because of these similarities, we can say that an accountant is the scorekeeper of a business.