Permitting a team to introduce a player after the match has started removes the oldest constraint in cricket selection. The consequences run well beyond the substitute himself.
Selection stopped being a single decision
Traditionally a captain names eleven players before the toss and lives with the choice regardless of how conditions develop.
With a substitution available, the team sheet becomes conditional. A side can name additional players and decide later which one the situation requires.
That converts a prediction about the match into a reaction to it, which is a substantial advantage in a format where the toss and the dew can dictate what is needed.
Both innings can be played with specialists
Under the old structure a team balanced batting depth against bowling options, and every extra batter came at the cost of a bowling choice.
A substitution allows an extra batter while chasing and an extra bowler while defending, so the same squad fields a stronger unit in each phase.
The visible result is deeper batting orders, with recognised batters occupying positions that used to belong to bowlers who could hold a bat.
All-rounders lost part of their premium
The value of a player who both bats and bowls came from solving the balance problem within one place in the eleven.
If the balance problem can be solved by substitution instead, a team can field two specialists rather than one player who does both adequately.
This is the most criticised effect, because domestic all-rounders get fewer overs and the pathway that produces them narrows.
Scoring rates rose for a structural reason
Batting sides can attack earlier when the order runs deeper, because the cost of losing a wicket falls when another capable batter follows.
Totals that once represented a strong innings became par, and chasing teams adjusted their approach in the same direction.
Bowlers absorbed the difference, which is why economy rates that would once have been respectable now look ordinary across a season.
Why the rule remains debated
Critics argue that removing the balance constraint removes a genuine skill of captaincy, and that the format is now weighted towards batting.
Supporters point out that a shorter format competes for attention and that scoring is what audiences reliably respond to.
Both positions accept the same underlying mechanism: the rule changed what a team is, and everything downstream followed from that.