The problem with unreported income is the domino effect. The hours worked on that job can't be reported, the guys then need to be paid off the books as well. When you or the guys apply for financing(mortgages, car loans, capital financing, etc...) this income can't be reported thus leaving you unable to get as much as you might have been able. If an employee is injured and out on disability or WC then their comp is 70% of the reported amount thus leaving them with much less than the 70%. The end result is more than just beating the tax man and insurance co's.
The nice thing about cash is there is no additional costs or delay to processing the payment. Many industries offer discounts for immediate payment as an incentive to get the cash in the accounts sooner. The strategy is to invest the money. The longer they have it the more they can make on it before it gets paid out to any of their bills. This is how big retailers make their money. They negotiate longer terms for payment on inventory, move the product quickly through deep discounts and third party financing. Take the cash invest it until the payments are due.