The purpose of a frontal is threefold.
(1) It is a covering of honour for the body of the altar which, as we have already seen from the liturgical books, represents Christ Himself; and if further proof is necessary, it is provided by the five crosses incised upon the upper surface of the altar, representing the five wounds in Our Lord's Body on the cross. Van der Stappen, Sacra Liturgia, ed. 2, vol. iii, Q. 42, i., says, "For the altar is Christ, therefore, on account of its dignity, it is clothed in precious vestments, as the Pontifical says in the ordination of sub-deacons." Moreover, on Maundy Thursday this frontal and the cloths are stripped off during the recitation of the psalm, Deus, Deus meus, in which the verse foretelling the parting of Our Lord's garments occurs...
(2)The frontal is a means of employing colour to bring out the full meaning of the very beautiful symbolism in that same office of ordination of subdeacons which speaks of "the faithful with whom the Lord is clothed as with costly garments." The red frontal, for instance, reveals the victory of the Rex Martyrum, realized afresh in yet another of His members...
Our Lord, as represented by His consecrated altar, puts on robes of majesty to identify Himself with those in whom His victory has borne fruit; His own purity reproduced again in the white robe of the virgin saint; His own heroic fortitude in the red robe of the martyr: and thereby additional emphasis is given to His invitation to be approach through the intercession of the saint with whose colour the altar is robed. And when the Robes of Majesty are all removed on Holy Thursday, more is indicated than the removal of His garments. His faithful, His costly garments, His disciples, are all stripped from Him; and His desolation is made the more evident by this complete annihilation of colour. But in addition to emphasizing the union of the Head with the saint commemorated by the feast of the day, the coloured frontal also serves to bring into clear prominence the union of the Head with His ministers of the altar, who are vested in the same colour...
The instinct to provide colour seems less vigorous today than in most other centuries, and one doubts whether its place is sufficiently realized as one instrument of a concerted orchestra, in whose harmony its omission forms a gap. Cardinal Schuster dwells on this accumulated harmony when describing the Introit for Whit-Sunday: "It is well known," he says, "that all the present texts of the Missal and of the Breviary have beautiful melodies attached to them. As no one, for instance, would desire to judge of an opera simply by reading the libretto of the author, but would wish also to hear the music and see the full effect of the mise en scène, so, in order thoroughly to appreciate the sense of beauty and inspiration, the powerful influence produced by the sacred liturgy on the Christian people, it is necessary to see it performed in the full splendour of its architectural setting, of the clergy in their vestments, of the music, the singing and the ritual, and not to judge it merely from a curtailed and simplified presentment."
(3) The frontal serves to give to the altar that architectural promimnence which its central position in the liturgy requires...
-- Geoffrey Webb, The Liturgical Altar
