Priests of the Society of St. Pius X live in community and go out from there to give divine life to souls.
The rule of the Society of St. Pius X states that its goal is essentially apostolic. It exists to save souls. Its founder, Archbishop Marcel Lefebvre, desired that the Society achieve this goal chiefly by the sanctification of priests, its own priestly members first of all.
The venerated founder of the SSPX also wanted his priests to live a common life. He knew that as men they needed the support and structure of a life lived with other priests.
Thus the reason for the priory. It is meant to be a place where the priest finds a necessary frame for his spiritual life as well as the helpful camaraderie of his colleagues.
Yet the priests do not only sanctify themselves; they do so to sanctify others. If their life has an emphasis on their community life, it is in order that they have a fruitful apostolate.
The priest exists to give the sacraments as they always have been given. The faithful have the right to ask for them as such. The priory established in Saint Marys exists to make available to the faithful the Catholic Mass and sacraments in all their fulness. It exists to give divine life to their souls, which Christ came on earth to give.