How precise you are as a pitcher determines if you will be a starter or reliever. Pitchers that maintain command and velocity longer in games will be starters. They are able to repeat their deliveries which will make their pitch location and speed consistent. Starting baseball pitchers know the importance of correct baseball pitching mechanics. Baseball pitchers who eventually become relievers show explosive pitches for a short period of time. They start to lose their effectiveness as the baseball game wears on because their pitching mechanics are sub-standard. The longer they stay in games the more their mechanics will break down.
Relievers also have shorter baseball careers because they burn out and lose their stuff as a result of stress on their arm caused by bad mechanics.
There are exceptions to almost every rule. Mariano Rivera of the New York Yankees, possibly the best reliever in history, has an outstanding release and great mechanics. But he started his career as a starter, and was switched to the bullpen out of need, much like Dennis Eckersley, a Hall of Famer.
Remember it is essential to learn and repeat the correct baseball pitching mechanics. It will maximize your pitching abilities, save your arm from injury, and give you the best chance for pitching success.