In the final round of a World Cup group, both matches begin at the same moment. The rule exists because knowing the other result creates an incentive to stop competing.
The problem simultaneous kick-offs were built to fix
If one group match finishes before the other starts, the two teams still playing know precisely what scoreline sends them through. That knowledge can make the contest pointless.
The damaging case is a result that suits both sides at once. Neither team has any reason to attack, and the match drifts to a conclusion that was effectively agreed before kick-off.
Nothing needs to be arranged for this to happen. Two teams simply acting in their own interest will independently arrive at the same passive outcome, which is why the fix had to be structural.
Uncertainty forces both matches to be played out
When the games run in parallel, no side knows whether the required result is still the required result. A goal in the other stadium can change what a team needs while it is on the pitch.
That uncertainty is the entire mechanism. A team defending a scoreline it believes is enough may find, minutes later, that it now needs another goal and has surrendered the initiative to get it.
Coaching staff track the parallel match from the touchline, but information arrives with a delay and cannot be acted on instantly. Uncertainty is reduced rather than removed.
Tiebreakers change what teams are chasing
Group placings are usually separated by points, then goal difference, then goals scored, with head-to-head results used in some competitions. Each rule changes what a team is actually playing for.
Under goal difference, a team leading comfortably still has a reason to score again, because margin can decide the group. Under head-to-head, that same margin may be worth nothing.
The choice of tiebreaker therefore shapes the last twenty minutes of a group stage as much as the scoreline does, and squads are briefed on it before the match.
Third-place qualification adds another calculation
Tournaments that advance the best third-placed teams from several groups introduce a comparison across groups that no team can control. A side may need a result elsewhere as much as its own.
Because those cross-group comparisons are settled by points and goal difference, a heavy defeat in a match already lost can still eliminate a team through the back door.
This keeps late substitutions and defensive shape meaningful even in dead rubbers, since the margin being protected is not about this group at all.
The rule applies unevenly by design
Simultaneous kick-offs are used for the final round and not the earlier ones, because the incentive to collude only exists once qualification arithmetic is fully visible.
Earlier rounds are staggered deliberately, since spreading matches across the day is what makes the group stage watchable and what broadcast agreements are built around.
The compromise accepts a scheduling cost for one round in exchange for removing the situation where two teams can agree on an outcome without saying a word.
