It seems better to me to narrate the history of that altar than to keep silent: this seems to be, or become, a stock phrase with which to introduce miracles associated with saints. For example, similar sentiments are expressed in the 12th C. Life of St. Cadog*
(English Translation) before the description of a miraculous fountain;
in the 7th C. life of St Eligius* (English Translation) in the pre-amble to
a section where the saint strikes down a sinner, and
in the 7th C. Life of St Columba*
(English Translation) preceding a section on a miraculous crane.
From a modern viewpoint it appears that there was an ambiguity about miracles, which might be thought in some way an unfortunate thing to dwell on unless instructive or glorifying - perhaps because they were too unbelievable. However this was not necessarily the case and the motives of the writers are probably lost to us. To an extent it may be modesty on the saints' behalf. St Patrick, in his own Confession* (English Translation), notes:
And there the Lord showed me my unbelief, that at length I might remember my iniquities, and strengthen my whole heart towards the Lord my God... Therefore I cannot and ought not to be silent concerning the great benefits and graces which the Lord has bestowed upon me in the land of my captivity, since the only return we can make for such benefits is, after God has reproved us, to extol and confess His wonders before every nation under heaven.
However, that this need not be the case either is revealed by an interesting passage from the 12th C. Life of St Patrick by Jocelyn* (English Translation) which has:
...the names of these holy prelates were Lugacius, Columbanus, Meldanus, Lugadius, Cassanus, Ceranus; but to mention the names of the bishoprics we for good reason omit—for in many instances we avoid the names of places and of persons, that we may not, by their uncouth barbarousness, occasion disgust or horror to cultivated ears. However, these prelates profited much the church of God by their conversation and by their example, and closed their lives in much holiness; for they were wont to relate many miracles to have been worked by the aforementioned seal-skin, the which even to this day remaineth entire, and is preserved as a relic in memory of Saint Patrick.
The general relucance to discuss a variety of religious matters seems to have been somewhat common and the reasons for this somewhat ambiguous.