The reason why those who are admitted to a monastery are not permitted to mix at once with the congregation of the brethren, but are first committed to the guest house.
When, then, any one has been received and proved by that persistence of which we have spoken, and, laying aside his own garments, has been clad in those of the monastery, he is not allowed to mix at once with the congregation of the brethren, but is given into the charge of an Elder, who lodges apart not far from the entrance of the monastery, and is entrusted with the care of strangers and guests, and bestows all his diligence in receiving them kindly. And when he has served there for a whole year without any complaint, and has given evidence of service towards strangers, being thus initiated in the first rudiments of humility and patience, and by long practice in it acknowledged, when he is to be admitted from this into the congregation of the brethren he is handed over to another Elder, who is placed over ten of the juniors, who are entrusted to him by the Abbot, and whom he both teaches and governs in accordance with the arrangement which we read of in Exodus as made by Moses.
Source: Institutes (New Advent)