The Diamondbacks have been calling up players to make their MLB debut in September since 1998. That’s since MLB rosters expand every September. Up until 2019, the MLB Active Rosters would expand to the full 40-man roster. Before the 2019 season, the MLB and MLBPA agreed to expand the normal roster to 26 players. Then, they agreed to expand September rosters to 28 players. The Dbacks have had some impacts made by players making their debut in September.
The reasoning behind the shortening of the Sept. roster was to quicken games and limit substitutions. Before the rule change, managers would put ten pitchers into one 9-inning game.