The derivatives are denominated in money, they cost money to purchase or to honor the contracts and they have been shown to have the ability to bring down the financial system because they create systemic risk.
In software terms, it's the equivalent of tight coupling of all your components. Instead of letting a buggy (e.g. insolvent) component (e.g. underwater mortgages) crash on their own, the tight coupling (via derivatives like CDOs and CDS's) brings down the whole system.