I often listen to the tennis matches that a left-handed has an advantage to play tennis.
What would be the reasons of this "skill"?
Lefties play a style of tennis that is almost mirror image to that of a righty. For instance, their forehand (usually their stronger shot) is on the side of the court where a righty's backhand (usually their weaker shot) would be. When they hit a slice serve it curves to the receiver's left rather than their right.
Since lefties are fairly uncommon in the game, players get accustomed to the way that righties' serves curve toward or away from them, and they feel safe hitting to certain areas of the court (usually the backhand side of their opponent). Players form these habits playing righties, to the point where it is almost unconscious. When they play a lefty, they have to mentally override these tendencies, which can be difficult and lead to more unforced errors.
Another commonly brought up advantage that a lefty has is that their better serving side (when they serve across their body) is the ad side. This is where most of the important points of a match are played from (since the only time you can win a game on the deuce side is at 40-15; you can win a game from the ad side at 40-0, 40-30, or any ad point from deuce onward).
To prepare for playing a lefty, professional players will often practice with a lefty the day before or the day of their match. This allows them to get accustomed to returning their serve and how to construct points in baseline rallies.
Another note on lefties serving to righties: When the lefty serves on the ad side (to a righties backhand), the returner must move further out since it harder to reach with backhand than with forehand on a service return. The righty doesn't have the same advantage when serving to a lefty (on the deuce side), because they have less experience doing it. Also jamming a returner with a serve to their body will naturally cause the returner to return to a lefty servers forehand, due to defensive habit. These body serves are particularly effective for a lefty on the first serve. In my experience players rarely practice with handedness in preparation for USTA competition. Such practice would be a good idea, but would require more commitment than a few days before a match. Instead, righties should find lefty friends to compete with for season after season. A righty needs to learn how to return the lefty serve and how to serve to a lefty. Left handed players often struggle playing other left handed players because of lack of experience too.