Solving the endless set
Before the tiebreak existed, a set had to be won by two clear games with no ceiling, which occasionally produced sets of extraordinary length once two players were both reliably holding serve. The tiebreak caps that risk at a fixed score, replacing indefinite extension with a single decisive game once a set reaches six games apiece. It trades one kind of fairness, letting a set run until a genuine gap appears, for another, guaranteeing a result within a predictable amount of time, and the sport has broadly decided the second is worth having.
A different rhythm of serving
Within the tiebreak itself the scoring and serve pattern change noticeably: players serve single points rather than full games, and the serve alternates far more frequently than in normal play. This compresses the effect of a single server's advantage, since no one player gets to lean on a run of protected points, and it puts a premium on winning points on the return, because those points arrive on a much shorter cycle than in a standard game.
Shadow effects earlier in the set
Because a tiebreak is a known, fixed-odds event, its approach changes behaviour well before it arrives. A player who fancies their chances in a tiebreak can afford to play a close set more conservatively, happy to let it drift toward six-all, while a player who wants to avoid one will take more risks in the games beforehand to force an earlier break. Reading a set's tactics without accounting for this shadow effect misses a genuine part of the strategy on show.