They do a LOT of good for the people in their communities, as the priest at Rosie's parish talked about, with wells, and practical things to help the people live their lives in a better, or at least easier fashion.
There are also lay folks who volunteer with the Maryknolls, and who live in other countries for a year or two, helping the priest in poor areas. We had a Youth Minister in our Parish who, with his wife and daughter, moved to South America to work with the Maryknolls. They came back the following year to do a Mission trip, for a few weeks, to raise money for the Maryknolls, but we were out of town and missed them.
I may be suspicious but IMHO if you engage Maryknoll folks in a political/religious discussion, they will explain why Catholics are obliged to support Barrack Obama, and that his position in favor of allowing the killing of newborns, if their parents do not want them to survive, is perfectly acceptable to the Magisterium.