The importance of left-handed relievers isn't what it used to be. Commissioner Rob Manfred's "three-batter rule", first implemented for the ultimately shortened 2020 season, significantly reduced a manager's ability (including Kansas City Royals skippers) to lean heavily on handedness when choosing which reliever to deploy in high-leverage situations.
The rule did not, however, totally eliminate southpaws as useful bullpen pieces. Its exceptions leave room for some use of lefty-lefty (or righty-righty, for that matter) matchups, and some big league clubs remain heavy on left-handed-dominated lineups.
Making sure their bullpens include one, if not more, lefties remains an imperative for general managers like KC's J.