The direction of prayer has had an enormous impact on the style of celebration. Priests celebrating toward the people can see if people are paying attention, like a public speaker or performer. If he is facing away from them, he has little idea of what is going on in the nave. Before Vatican II, this was regarded as a good thing: in the 1962 Mass, even when a priest is told to turn to face the people (for example, when he turns to them to say, “Dominus vobiscum”), he should keep his eyes lowered. He should not be distracted by the congregation from his work of prayer, and the congregation should not be distracted by him.
Where celebration versus populum is the norm, Catholics can get used to priests trying to engage them in what is going on with eye contact, emphasis in what they are saying, and even unscripted remarks. Experiencing Mass celebrated the other way can even be disconcerting. The point of it, however, is to allow the people to do what the priest should be doing: addressing their prayers to God. Above all in the Eucharistic Prayer, a celebrant facing East allows the people to follow what is going on in their own way, and to unite themselves spiritually to his prayer to God, which he leads from the altar.
Read full article by Joseph Shaw here.