Certified cars tend to be inspected, refurbished, and certified by the manufacturer so the dealership can sell the car "like new". A dealership can spend the money to get a car certified because it benefits both the dealership and the consumer. Dealerships can justify the higher price because it's certified, can guarantee the car won't have any unknown issues, the car can be aligned, have new tires, brakes rotors if needed, etc, and can also sell you things like an extended warranty that applies to certified cars. The consumer will have piece of mind buying a certified car.
To qualify as a certified car the car must meet requirements, generally something like no more than 4 years old and no more than 75,000 miles, so that 2008 you mentioned is already too old. Sometimes a dealership will auction a car off if they cannot sell it, or if the car has too many problems that fixing it then selling it would result in a loss.