27 All these things, though necessary for everyone who is sent to the nations, can scarcely be attained to in reality by individual missionaries. Since even mission work itself, as experience teaches, cannot be accomplished by lone individuals, a common calling has gathered these individuals together into institutes, in which, with united efforts, they are properly trained and might carry out this work in the name of the Church and under the direction of the hierarchy. For many centuries, these institutes have borne the burden of the day and the heat, devoting themselves to missionary labor either entirely or in part. Often vast territories were committed to them by the Holy See for evangelization, and there they gathered together a new people for God, a local church clinging to their own shepherds. With their zeal and experience, they will serve, by fraternal cooperation either in the care of souls or in rendering special services for the common good, those churches which were founded at the cost of their sweat and even of their blood. Sometimes, throughout the entire extent of some region, they will take certain tasks upon themselves; e.g., the evangelization of groups of peoples who perhaps for special reasons have not yet accepted the Gospel message, or who have thus far resisted it. If need be, let them be on hand to help and train, out of their own experience, those who will devote themselves to missionary activity for a time. For these reasons, and since there are still many nations to be led to Christ, the institutes remain extremely necessary.
Source: Ad Gentes (Vatican.va)