Our Divine Redeemer had no need, as St. Ambrose says to retire to a lonely place to pray; for as His blessed soul was constantly in the presence of His heavenly Father, in every place and in all circumstances He thought of Him and continually interceded for us. Nevertheless, as St. Matthew relates, "Having dismissed the multitude he went into a mountain alone to pray. And when it was evening he was there alone." (Matt. 14:23). He did this to teach us the necessity of interior prayer or meditation.
The eternal truths are spiritual things; they cannot be seen with the bodily eyes, but only with the eyes of the soul, that is to say, by reflection and meditation. It is for want of reflection that, as the Holy Ghost says, "all the land is made desolate because there is none that considereth in the heart." (Jer. 12:11). During interior prayer the Lord speaks to us and enlightens us. "Thy word is a lamp to my feet and a light to my paths." (Ps. 118:105).
St. Bernard compares mental prayer to a mirror, and the comparison is very apt; for if you happen to have a speck of dirt on the face and you come before a looking glass, you see the dirt at once and remove it. Had you not looked in the mirror, you would not have thought of the dirt nor washed it away. So it is with mental prayer: While at meditation we are standing, as it were, before a mirror of the soul. It is then we recognize our faults and the danger we are in, and accordingly we take measures to rid ourselves of the faults and to escape the danger that threatens us. "He," says St. Bernard, "who does not meditate will scarcely ever perceive his faults, and as a result will have no horror of them." Without meditation or mental prayer we are lacking in strength to resist the assaults of the enemies of our soul and to practice the Christian virtues.
(Parts taken from: The 12 Steps to Holiness and Salvation)