A stale rental occurs when a rider forgets to end their ride after they've reached their destination. Finding and refunding a portion of a rental will eat into your ops team's time. Every minute a vehicle is in a stale rental it is not available to another user, cutting into your revenue.
Spring helps mitigate stale rentals by alerting the rider that they should end their rental if they haven't moved recently, and by alerting the ops team that they should keep an eye on the rental. A fresh fleet is a healthy fleet!