Some people may think "Oh I have air bags, that'll be enough." Well unfortunately that's wrong, your air bags are designed to work with your seat belt. If you're not wearing your seat belt you could be hurled into your air bags causing injuries or death. Without your seat belt there is also a chance you will be thrown from the car completely which almost always results in serious injuries or death.
Some people may think "Oh it's a quick trip, just running to the store I don't need my seat belt." Most fatal accidents happen within 25 miles of your home with speeds under 40mph. Any trip in your car, whether it be 3 blocks away or 3000 miles away can result in a crash, it's best to just always wear your seat belt.
For kids it's not enough to just wear a seat belt, make sure kids are in the recommended car seats or booster seats for their age, weight, and height!
Head over to https://www.nhtsa.gov/risky-driving/seat-belts to read more about seat belt safety.