Length as a filter against luck
Requiring three sets to win rather than two means a single bad patch, a lucky net cord or a brief lapse in concentration is far less likely to decide the outcome. Over five possible sets there is simply more room for the better player on the day to recover from an early setback, which is why the longer format is traditionally reserved for the stage of a tournament considered to need the most reliable result. It is, in effect, a deliberate choice to trade shorter, higher-variance drama for a longer, more accurate test.
A different physical and tactical test
Beyond fairness, the extra length changes what wins. Matches that can run to five sets place a much higher value on physical conditioning, since a player who tires by the fourth set has no format-based shortcut to compensate. They also reward tactical patience, because a set lost early can be treated as recoverable information about an opponent rather than a crisis, whereas in a two-sets format the same lost set can be nearly fatal to a player's chances.
Same rivalry, different favourite
Because of these differences, it is entirely possible for one player in a rivalry to be favoured in a three-set match and the other to be favoured over five, particularly when fitness, movement efficiency or a slow-starting habit are involved. Recognising which format is in play is therefore not a formality; it is one of the first pieces of context worth applying before predicting how a match between two known players is likely to unfold.