To help the individual monastic to persevere in his or her vocation.
St Antony the Great, one of the first monks in Egypt, had a vision in which an angel instructed him not to try and pray all the time, but to alternate prayer with a routine work which would let the mind rest. This is very good psychology. Monastics do manual work, as distinct from intellectual work, in order to balance mind and body and to keep both healthy. Also, doing work which requires physical effort yet little thought can reveal to the monastic what is lurking in his or her psyche, because in the process many things can arise from the subconscious which need to be prayed about.
To help the Church in her pastoral outreach.
In missionary lands where there is little or no pastoral structure, monasteries have been extremely effective centres of evangelisation because of their communal witness to higher realities, which can be more important than the preaching of individuals. Throughout history the Church has made use of monastics in building up her pastoral strength, but it must be said that the monastic life is not primarily intended to be involved in the developed direct apostolate after the missionary thrust has borne fruit.
There can be a real contradiction between the life expected of secular priests and the active sisterhoods, for example, and that necessary for monastics. The latter are called upon to live in community in one place, which is their permanent home. The Church in the last forty years has put great emphasis on the need for monastics to be faithful to this primary calling, and to be wary of any outside demands that may compromise it.