This is a function of cash flow into a business as well as an indicator of how well a business is doing. More specifically, there are two ways that this can affect a business. The first is to show a given amount of sales on the books while not having the same level of liquid revenue at hand. The other is to have a balance between the amount of sales and the revenue received, which increases the liquid assets of a business, (liquid assets, in this instance, being revenue on hand, or in the bank).
Reporting all incoming cash flow to a bookkeeper helps to stay on top of what is coming in and what is expected. Furthermore, because this is where the earned revenue of a business materializes, it is necessary to have accurate records of the transactions that generate revenue at tax time.