The times-through-the-order problem
A hitter facing the same pitcher for the third time in an afternoon has seen the pitches, the release point and the sequencing. Familiarity is an advantage, and it accumulates within a single game. Once that pattern was widely accepted, the question stopped being whether a starter was tired and became whether the lineup had learned him.
That reframing is what changed managerial behaviour. Removing an effective pitcher looks strange if you think of fatigue as the only reason to act. It looks obvious if you think the next nine hitters have a better plan than the last nine did.
Specialisation follows
If a pitcher only has to record three outs, he can throw at maximum effort with two pitches rather than pacing himself with four. Rosters filled with arms built for that assignment, and the late innings became a sequence of short, high-intensity confrontations rather than a war of attrition against one man.
This also reshaped how teams are built. Depth in the bullpen became a competitive asset that could be assembled relatively cheaply compared with a front-line starter, which is exactly the kind of inefficiency organisations chase until it stops being an inefficiency.
The costs the sport keeps debating
Every change of pitcher is a pause, and pauses accumulate into a longer, more fragmented evening. The rules changes of recent years are largely an argument with that consequence rather than with the underlying strategy, which remains correct on its own terms.
There is a competitive cost too. A manager who goes to the pen early is betting that three relievers will out-perform one starter, and the bet fails often enough to be interesting. When it fails it is highly visible, which is why the tactic is criticised more than its record deserves. Arena's MLB rules and positions guides explain the mechanics behind these decisions.