Baseball needs facial hair. Nothing is more boring than watching a parade of shaved clones (often with shaved heads) inning after inning. We need variety, and that’s what Reggie and the rest of the A’s gave us at the dawn of the 1970s.
Here’s what Wikipedia has to say about the Oakland experiment that ended the historic taboo for mustaches (and beards) on the diamond.

The Origins
Baseball, before 1972, was traditionally known as a conservative era where all players were clean shaven. During this time there had even been an unwritten rule that frowned upon players with facial hair. There have been some baseball players who have grown mustaches but all showed up cleanly-shaven at the start of the regular season either by their own decision or as ordered by their managers.

The Mustache Gang was started in 1972 when right fielder, Reggie Jackson, showed up to spring training with a mustache claiming he would have a fully grown beard at the start of the regular season. Initially, this assertion was not taken well by the organization and according to Mike Hegan, “[Charlie] told Dick to tell Reggie to shave it off. And Dick told Reggie to shave off, and Reggie told Dick where he could shove it.” [3] So, in a hopes to avoid a large conflict Finley decided to take a reverse-psychology approach, knowing Reggie Jackson thought of himself as an individual, he hoped that if a couple other players decided to join him in sporting facial hair then he would give up and shave off his beard. In an attempt to do just that, Finely asked A’s pitchers Jim “Catfish” Hunter, Rollie Fingers (who came to be known for his long handlebar mustache), Darold Knowles, and Bob Locker to all grow a mustache.[4] This backfired and lead to the birth of The Mustache Gang.

As the mustache spread in popularity among the team, Charlie O. Finley and the other management began to come around to the new look. Finely had come around so quickly, he not only grew one but encouraged the other members of the team to do the same. He even dubbed that year’s “Father’s Day” as “Mustache Day” and offered a $300 reward to anyone who could grow a mustache by then. Once “Mustache Day” rolled around, all 25 members of the Oakland Athletics was sporting a mustache, even Dick Williams decided to grow one;[5] to further promote his team’s new look, every fan that showed up to the Oakland Stadium wearing a mustache gained admittance for free. As though to irritate the other owners even more, Finely took it one step further by making his team’s uniforms different from the rest of the league, the traditional baseball pants and black and grey jersey was replaced with a “two-tone uniform”. Meaning, the players would wear a green and gold pullover shirts on top, along with the traditional white pants and elastic belts.[1] This created a baseball team unlike any other in its time.