The number of substitutions a manager may make sounds like an administrative detail. It determines how hard players can run and how a squad is built.
Substitution began as injury cover
Changes were originally permitted so that a team reduced by injury was not forced to play short, and tactical substitution was a secondary use of the allowance.
With only one or two changes available, a manager had to hold at least one back in case of an injury late in the match.
That reserve requirement meant tactical changes were rare and usually late, and the eleven who started expected to finish unless something went wrong.
More changes means higher intensity
A player who knows he will play the full match paces himself, because sprint capacity is finite and depletes over the course of ninety minutes.
A player who expects to be replaced can spend everything he has, since the recovery cost falls on someone sitting on the bench rather than on him.
Increasing the allowance therefore raises the average intensity of the whole match, which is why pressing schemes became more sustainable once more changes were permitted.
The bench becomes part of the plan
Managers now select substitutes for defined jobs rather than as generic cover, naming a player to hold a lead and another to chase a goal.
Changes are increasingly made in pairs at scheduled points, refreshing an entire unit at once so that the shape stays coherent rather than patched.
This has moved substitution from a reaction into a pre-match plan, with a rough timetable agreed before kick-off and adjusted only if the score demands it.
Squad depth becomes a structural advantage
When five changes are available, the gap between a club's eleventh and sixteenth best players matters far more than it did under a two-substitute rule.
Wealthier clubs can field a strong bench, so a rule intended to protect player welfare also widened the practical distance between rich and poor squads.
Some competitions have responded by limiting the number of stoppages in which changes may be made, restricting the disruption without reducing the allowance itself.
The welfare argument behind it
Fixture calendars have grown, and the same players are asked to appear in more competitions across a season than any earlier generation faced.
Allowing more changes distributes minutes across a squad, which reduces the cumulative load on individuals and lowers the injury risk that comes with it.
The rule is defended primarily on those grounds, and the tactical consequences are a side effect that coaches have taken full advantage of.
