Wearing a seat belt is more that just good practice for safety while in a car. It can also affect whether or not you can successfully press a claim if you do wind up injured. That can be an unpleasant surprise in the aftermath of an incident.
When a person gets injured in a car accident, passenger or driver, they have the right to file a claim with their insurance company to attempt to get compensation. The claim can include details of the damage to the car as well as any injuries. The insurance company will investigate the claim to determine responsibility. They will reduce or deny the claim if they find that the incident was the fault of the person filing the claim. In the case of a seat belt, wearing a seat belt is a fundamental part of car safety, and it is also required by law. As a result, if a person is injured but was found to be not wearing a seat belt at the time, the insurance company can argue that their injuries were the result of their own choice not to wear it.
The injuries that can arise from not wearing a seat belt are often particularly painful and long-lasting, because they are often injuries to the back and neck. Car accidents are bad enough already, but having the insurance claim compromised so that you have to pay for the medical care afterward makes things that much worse.
It is important to know that not wearing a seat belt can bias an insurance company against your claim.