It's the acceptance that no justice system can be 100% perfect. As repeated in many other comments, it gives the innocent person a chance to be released, which

It's the acceptance that no justice system can be 100% perfect. As repeated in many other comments, it gives the innocent person a chance to be released, which is better than the zero chance. Also as you say - 20 years in jail is a serious punishment, so it isn't eliminating the punishment, it's just only using a reversible one. It also allows for improvements in the justice system over time.