The canopy, according to Canon J.B. O’Connell, “was a traditional mark of reverence and honour, emphasising the royal dignity of the altar...without any infringement of the inviolable sanctity and detachment of that sacred stone.” Canon J.B. O’Connell, Church Building and Furnishing: the Church’s Way, 1955, pp. 185-86.
The most famous example of such a canopy is in Saint Peter’s Basilica; but there are numerous such canopies throughout the United Kingdom, Europe, North America and in Australia. Many of these were built in the first half of the twentieth century, when liturgical ideals were being practically espoused.
The former Ceremonial of Bishops and various decrees of the Congregation of Sacred Rites required a canopy of some sort to be built over an altar - a directive which, unfortunately, was largely ignored.