The payables clerk handles all outgoing cash flow related transactions. All company bills are handled by these individuals. Their overall impact lies in their ability to apply accuracy and due diligence in regard to your company's expenses.
These individuals are responsible for optimizing the outflow of cash (minimizing as much outflow as possible--within reason of course). This covers small detailed actions that can result in extra costs vs. costs savings. So, though you as the player can see the overall amount of company expenses and even some itemizations from a macro standpoint; this team actually looks at each incoming invoice, etc...(micro-scale). There are simply too many factors that can result in higher cost or, preferably cost savings that you the player just cannot handle yourself.
As these individuals handle these transactions directly, they can have a huge impact on your company's bottom line.
Alternative to the payables role, the receivables clerk handles all incoming cash flow to the company. All payments from customers, other companies and organizations get handled by these individuals. As the payables clerk can impact the bottom line, the receivables clerk is just as important. Also, it might be good to have a good background checking system at your company. You do not want funds to go mysteriously missing.
These individuals are responsible for making sure that you receive all of the payments that you are owed.