McCann presumably lost his starting job when the White Sox signed Yasmani Grandal, and was suspected by many to be a non-tender candidate, but they chose to keep him in house as a backup and someone who can spell Grandal and allow him to DH on occasion.
McCann had his best overall offensive season last year, hitting .273/.328/.460 with 18 homers, but it was a tale of two seasons for the guy. Before the All-Star break he hit .316/.371/.502. After the break: .226/.281/.413, with a spike in his strikeout rate and a reduction of his walk rate. In his five previous seasons McCann hit a combined.240/.288/.366, so those second half numbers in 2019 seem far closer to the mark.
Maybe the Sox think they know something that make them think that first half was some new level for the guy. Maybe they just like him and want two catchers who could start for most teams on the roster.