Generally speaking, lefties have a batting advantage against right handed pitchers, and of course, there are more right handed pitchers than lefties. Also, left handed batters are closer to first base and have a slight time advantage in reaching first base after a hit. Left handed pitchers have an advantage in being able to watch a first base runner attempting to steal second.
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You can swing the ball wide to their backhand when serving from the deuce court, your opponent has to hit to the opposite side that they normally do to get it to your backhand, and there is a slight psychological advantage because many players are not used to playing against lefties and they think that being left-handed gives you a bigger advantage than it actually does.
A left handed pitcher can get a curve ball to break more than a right handed pitcher throwing a screw ball against a slap hitter hitting left handed.