Sweeney had not been on the 40-man roster and had to be added; the DFA move for Loe opened the spot for Sweeney, while Sappelt, with options, remains on the 40-man while he plays in Des Moines.
Dolis, like seemingly so many pitchers coming through the Cubs system, has a great arm and throws hard, but can't seem to throw enough strikes to stay in a major-league bullpen. He's been on the roster already once this spring, throwing one-third of an inning in the 10-7 loss to the Giants April 14. Dolis came into the game after Shawn Camp had already given up the lead, and retired the only batter he faced to end the 10th inning.
This could be a temporary stay for Dolis until Kyuji Fujikawa returns, or, if he can do well enough, Camp might be the next one to exit the revolving bullpen door.