Mid-table stops being dead
In a conventional league season, a club that cannot reach Europe or fall into relegation trouble has weeks of fixtures with nothing at stake, and everybody involved can feel it. Play-off systems attach jeopardy to positions that would otherwise be inert, which means a seventh-place finish is worth fighting for and a run of April draws has consequences. For supporters, that is straightforwardly better than a month of exhibition football.
For coaches it changes planning. Rotation in March is no longer a free choice, because the reward for the season may be decided in two-legged ties played after everyone is tired.
Depth and fitness decide short series
Knockout football at the end of a season favours squads that still have available players. A team carrying two long-term injuries in the same position, or one whose first eleven has played almost every minute, is exposed by the compressed schedule in a way the league table never reveals. That is why clubs that manage minutes carefully through the winter often outperform their league position once the ties begin.
It also raises the value of players who are unremarkable over thirty-four matches but useful over two: a substitute who can defend a lead, a set-piece specialist, a goalkeeper who is calm in a shoot-out.
The fair objection
The strongest argument against play-offs is that they can overturn the verdict of a whole season, letting a side that finished several places lower take the prize on the basis of two good nights. That is a real cost, and no amount of extra excitement makes it disappear. Leagues adopting these systems are making a trade: more meaningful fixtures in exchange for a less pure ranking.
Whether the trade is worth it depends on what you think a league is for. If it exists to identify the best team, the objection wins. If it exists to produce a season people follow to the end, the format has a case.