A lot can change in just one at-bat in a baseball game. A lot can change in a whole season if that one at-bat comes in the playoffs, especially in the World Series.
Win Probability is great for showing just how much changes in an important at-bat within a game. It also shows how much an impact someone can have even if they only make a brief appearance in a game. While it’s usually unlikely that a pinch hitter can have the most hits for his team, and a reliever probably won’t lead the team in strikeouts, they often are tasked with coming in for some of the highest leverage moments in the game.