There's a classic Simpsons episode in which the sly businessman Mr. Burns recruits real Major League Baseball players to join his company softball team in order to win a bet. But when the championship is on the line, Mr. Burns pulls eight-time National League all-star Darryl Strawberry for a substitute, Homer Simpson. "You're a left-hander, and so is the pitcher. If I send up a right-handed batter, it's called playing the percentages," Mr. Burns says to Strawberry. "It's what smart managers do to win ballgames."
Lots of folks think Win Probability Added (WPA) is a useful statistic. Many coverages, often including our own game wrap polls, like to refer to it. Others think it's just something dreamed up by a FanGraphs intern in 2008 in order to at least have something to put on a resume and has little purpose for anyone who can easily figure out the important moments in a game's flow without it (raises hand, looks around to see if anyone is nodding in agreement).