If you have vehicle insurance, it is unlikely that you will be fully reimbursed if you have an accident. What is the motivation for deductibles and co-pay? If accidents happen by pure chance, surely it would be better to have insurance contracts that perfectly pool risks and thus relieve you of all the losses associated with a particular accident. But full insurance invites moral hazard: if we are fully insured, we may become more careless.
The tension between insurance and incentives is due to a combination of two factors. The first is a conflict of interests: not everyone is an angel. If we were all equally careful, regardless of whether or not we bore the full consequences of our behaviour, full insurance would be unproblematic. The second factor is measurement: not all our actions can be perfectly observed. If an insurer could see every careless action, an insurance contract might fully cover all losses caused by true accidents, but not those caused by reckless behaviour.
The same type of tension may be present in many other contractual settings, like employment relationships. In most cases, an employer is better able to bear risk than an employee. If the employee were always to act in both parties’ best interest, no incentives to the contrary would be needed: the tension between insurance and incentives would be moot, and it would be optimal to offer the employee the insurance of a fixed salary. But if his interests clash with those of the employer and his behaviour is difficult to observe directly, an employment contract with a stronger link between pay and performance may be desirable.