The referee is named days in advance and mentioned in almost no previews. Officials vary enough in how they apply the laws that the appointment interacts directly with how a match will be played.
Interpretation varies within the laws
The laws leave room for judgement on contact, on what constitutes a clear scoring opportunity and on when persistent infringement becomes a caution.
Referees develop consistent tendencies within that room, and those tendencies are visible across a season in their foul and card rates.
None of this is error. It is the inevitable result of asking humans to apply written rules to fast, ambiguous events, and the alternative would be a set of laws too rigid to referee.
The interaction with playing style
A physical team that defends with contact benefits from an official who lets play continue, and suffers under one who whistles early.
A technical side that draws fouls in midfield gains from a strict interpretation, because the fouls that break up its possession get punished.
The same appointment therefore favours one team and disadvantages the other, and which way it points depends on the fixture rather than on the referee.
Card thresholds change substitution plans
A coach whose defender is booked early under a strict official faces a decision that would not arise under a lenient one.
Early cautions frequently force a substitution before the hour, which removes a change the coach had planned to use tactically later.
Teams in European competition brief specifically on this, because the official comes from a different national tradition and the tolerance level may be unfamiliar.
Video review shifted where decisions are made
Review systems concentrate scrutiny on a narrow set of decisions, and the on-field official's threshold still determines whether many incidents are examined at all.
Penalty rates changed noticeably once contact in the box began receiving frame-by-frame attention, which altered how defenders position their arms and bodies.
The protocol also lengthens stoppages, which affects added time and the closing minutes of tight matches.
How much weight to give it
The effect is real and small, comparable to a minor injury absence rather than to a change of coach.
It matters most in fixtures where the two teams have sharply contrasting styles, since that is where a tendency has something to act on.
Checked in thirty seconds and weighted lightly, it is one of the better returns available in match preparation. Treated as a decisive factor, it becomes the excuse it is so often used as afterwards.