Simply because they now have leverage over future payouts. If the amount outstanding is significant to you (say maybe 1/12th of your annual income) then the cli

Simply because they now have leverage over future payouts. If the amount outstanding is significant to you (say maybe 1/12th of your annual income) then the client can do things like: "We're really close to being able to afford to pay your bill, but I won't be able to do that until you give us the new features we talked about." So now you're at a crisis point where you have to decide if you're going to keep putting in "free" work with a client that owes you money in hope of future additional payout. Whereas if the guy is just late on a day or two of pay and you can say "I can't do anymore work until you've paid your invoice", then you can walk away free and clear with little lost sleep. It's like that old saying (rough paraphrase): If you owe the bank $10,000, it's your problem. If you owe the bank $10,000,000, it's theirs. :)