From the Toronto Star - link
Players on the disabled list still earn MLB service time, moving them closer to free agency. Travis has been on the DL four times, for a total of 271 days, more than half of his major-league time; if he stayed on the MLB roster or DL, he would have been eligible for free agency at the end of the 2020 season.
If the Jays keep him in the minors for at least 15 days, they will then have him under club control through the 2021 season. That, of course, means Travis will likely remain with the Bisons until at least May 15 when the Jays travel to play the Mets, perhaps even longer.