Also, mastery of a cutter effectively gives you mastery over hitters for a longer period of time. If you can locate a good cutter regularly, you effectively give life to your fastball even as you age. Pettitte and Rivera are the biggest examples of that. Rivera still throws a low 90s cutter with monster movement. Pettitte throws a mid 80s cutter with good movement, but he uses it to highlight his upper 80s fastball and makes it look a lot harder. It is a method to prolong your career, but it shouldnt be the first thing 25 yr olds are learning at the big league level. They should learn it when they are 33 and start losing velocity. By that point, they will have mastered their deliveries and can learn the pitch without having to fight their mechanics