A substitution looks like a judgement on the player leaving the pitch. It is usually a decision about the shape of the team, and the individual is the means rather than the reason.
The change alters the structure, not just the personnel
Removing a midfielder for a forward shortens the distance between attacking players and lengthens the distance the remaining midfielders must cover.
Coaches therefore plan substitutions in pairs, deciding what the shape will be afterwards before deciding who will leave.
A change that improves one area while destabilising another frequently makes the team worse despite bringing on a better player.
Fatigue is positional as much as individual
Wide players and full-backs cover the most ground and accumulate the most high-speed running, so they fatigue earlier regardless of fitness.
A team's pressing intensity falls first in those positions, which is why the first substitutions in many matches are wide players rather than underperforming ones.
Staff use tracking data alongside observation, but the visible signal is usually the same: a player who stops making the second recovery run.
A booked player is a constrained player
A defender or holding midfielder carrying a caution cannot commit fully to duels or make a tactical foul, which removes part of his function.
Coaches weigh that reduction against the disruption of a change, and often substitute the player at the next natural break rather than immediately.
The decision becomes near-automatic in a tight match where the tactical foul is a tool the team expects to need.
The scoreline changes what the bench is for
When chasing a goal, substitutions add attacking players and accept structural risk, since a draw and a defeat carry a similar cost.
When protecting a lead, changes aim at fresh legs in the positions that hold the shape rather than at extra defenders.
Coaches usually decide the general plan before the match and adjust only the timing, because the decisions are poor when made under late-game pressure.
Timing is a variable in its own right
Changes made around the hour mark give a substitute enough time to influence the match; changes in the closing minutes are largely about time management.
Making all changes at once resets the whole team's rhythm simultaneously, which some coaches use deliberately after a poor first half.
The alternative, staggering them, keeps a continuous supply of fresh players without ever disrupting the structure heavily, and suits teams whose plan is already working.