Example use case:
You've built a project targeting 30FPS, but it turns out to be optimized enough to run at 60FPS.
Normally, switching from using _update to _update60 after the fact would be a painful job involving a lot of manual number editing, and you'd be screwed. However, if you had built your project with delta time in mind from the start, you could simply rename and everything will continue to move at the 'correct' rate.
Made as an exercise in tool-learning and object-oriented programming.
Updated with sounds, refined physics and a couple of new mechanics. Bounce the ball off the ceiling or floor for style!
Properly constrained paddle to play area.