wRAA measures how many runs a hitter contributes, compared with an average player -- so a player with a 0 wRAA would be considered league average, offensively. It's calculated by finding the difference in the number of runs contributed between a player and the league average (which is determined by the league average wOBA).
Because wRAA uses wOBA to determine how many runs a player is worth, a player with an above-average wOBA will have an above-average wRAA. But -- unlike wOBA -- wRAA is a counting stat. As a result, players with a higher number of plate appearances can accrue a higher wRAA than an equal player with fewer plate appearances.
((wOBA - wOBA of the entire league) / annual wOBA scale) x PA
Watch: Bryce Harper belts three home runs in a single game, providing a massive boost to his wRAA.
Why it's useful
Obviously, a hitter's goal is to contribute runs in any way possible. wRAA measures how well players do so in relation to the rest of the league. And because it's scaled to the league, you can compare players on different teams or from different seasons.
Like wRC+, wRAA is a useful tool for evaluating offensive performance in a vacuum. But fantasy leagues aren't played in a vacuum, and adjustments aren't made for league or ballpark, so beware.