Back in 1973, the American League adopted the “Designated Hitter” effective for the upcoming season. It meant that the lineup would include a hitter instead of the pitcher. The National League did not go along. So major league baseball was played with different rules until both leagues adopted the DH in 2020.
The new rule went into effect when Luis Tiant of Boston was the first AL pitcher to face a DH or Ron Bloomberg of the Yankees on opening day 1973. The DH impacted veterans and allowed pitchers to stay in the game, i.e. no pinch hitter late in the game when the pitcher came to bat.
Three-time batting champion Tony Oliva, who could no longer play the outfield every day, swung the bat with authority: .291 with 16 HR & 92 RBI as a full time DH in 1973. Tony-O followed that with a .285 average in 1974. The DH rule allowed Tony-O to pass 2,000 hits.