Possession teams are often described as playing quickly, but the distinguishing feature is variation. The slow passages exist so that the fast ones have something to break.
Constant speed becomes predictable
A team that circulates at one tempo allows a defensive block to settle into a rhythm, sliding across at a matching speed and never being caught out of position.
Defending becomes considerably easier when the timing of the next pass can be anticipated from the last twenty.
Changes of pace are what break that anticipation, and a change requires a slower baseline to accelerate away from.
Slow circulation is reconnaissance
While the ball moves laterally at low speed, players ahead of it are moving into position and the team is reading how the block is arranged.
The opponent's structure is revealed by where it refuses to be pulled, and that information determines where the acceleration will be aimed.
A team that has already decided where to attack does not need this phase, which is why counter-attacking sides can skip it entirely.
The acceleration is a designed sequence
The change of speed typically involves a vertical pass into a forward, a lay-off, and an immediate third pass into the space the defence has just left.
Because the sequence is rehearsed, the players involved move before the first pass is played rather than reacting to it.
The defensive block, having settled into the slower rhythm, is a step behind for the two or three passes that matter.
Tempo also manages the physical cost
Sustained high-speed circulation is demanding for the team in possession, not only for the team chasing it.
Slower passages allow players to recover while retaining the ball, which is why possession is sometimes described as a form of rest.
Teams late in a demanding schedule frequently reduce tempo for long stretches, accepting fewer chances in exchange for lower load.
The instruction has to come from somewhere
Tempo is usually set by one or two players, typically the deepest midfielder and the goalkeeper, whose decisions on when to release determine the speed of everything ahead.
A midfielder who plays every ball first time removes the team's ability to slow down; one who always takes two touches removes its ability to speed up.
The players trusted with that role are chosen for judgement rather than technique, since the technical requirement is modest and the decision-making requirement is not.