The real target is substitution
The common description of tempo offence as simply playing fast undersells what it actually attacks. Substitution rules require a defence some window to change personnel, and a team that snaps the ball quickly can deny that window entirely, forcing a defensive unit built for one situation to face a completely different one. The advantage is not extra plays for their own sake but the mismatch created when a defence cannot respond to what the offence has just shown it.
A trade, not a free advantage
Tempo carries real costs. An offence playing fast tires its own line and skill players, and a short possession that fails still hands the ball back to an opponent quickly, which can expose a defence to a compressed, tiring workload of its own. Coaches who commit to tempo are making a considered trade: sacrificing some control and stamina for the chance to catch a defence in the wrong grouping. It is a strategic bet, not simply an aesthetic choice about how exciting a game should look.
How defences answer without matching speed
Rather than trying to substitute as quickly as the offence snaps, many defences respond to tempo by simplifying their calls in advance, using fewer distinct fronts and coverages so that whichever personnel happens to be on the field can execute the plan without a mid-drive adjustment. That trade-off between complexity and speed is one of the more interesting quiet battles in the sport, and it rewards defensive coordinators who plan for tempo before kickoff rather than reacting to it during the game.