Meditation is necessary so that we may have  Light  on the  Journey  to  Eternity.
"The mouth of the just shall meditate wisdom, and his tongue shall speak judgment.
The law of his God is in his heart,and his steps shall not be supplanted (Ps. XXXVI, 30-31)".
He who practices meditation is "like a tree which is planted near the running waters,
which shall bring forth its fruit in due season (Ps.I,3)".

    Meditation or mental prayer is necessary in the first place, in order that we may have light on the journey we are making to eternity.  The eternal truths are spiritual things, which are not seen with the eyes of the body, but only in the mind by consideration.  He that does not meditate, does not see them;  therefore he walks with difficulty on the way of salvation.  And further, he who does not meditate, does not know his defects, and therefore, says Saint Bernard, he does not detest them.  So also, he does not see the dangers to salvation in which he is, and therefore does not think of avoiding them.  But when any one meditates, his defects and the dangers of losing his soul at once come before him; and seeing them, he will seek to remedy them.  St. Bernard says that meditation regulates our affections, directs our actions, and corrects our defects.

    In the second place, without mental prayer we have not the strength to practice virtues and resist temptations.  God is most willing to give us His graces; but Saint Gregory says that before giving them, He desires to be asked, and, as it were, compelled to give them through our prayers.  "God  wants to be urged; He wants to be overcome by a certain importunity".  But without prayer we shall not have strength to resist our enemies, and so we shall not obtain perseverance in virtue.  

    Bishop Palafox, in his note upon the tenth letter of Saint Teresa, writes thus:  " How will God give us perseverance, if we do not ask for it?  And how shall we ask it without practicing mental prayer?"  But he who practices meditation is "like a tree which is planted near running waters, which shall bring forth its fruit in due season (Ps.I,3)".

    And further, meditation is the blessed furnace in which souls are inflamed with divine love:  "In my meditation," says the Psalmist, "a fire shall flame out."  Saint Catherine of Bologna said:  "Meditation is that bond which the soul to God."  In the Sacred Canticles it is written: "The King brought me into the cellar of wine, he set in order charity in me (Ct.II,4)".  This wine-cellar is meditation, in which the soul becomes so inebriated with divine love, that it loses, as it were, its feeling for the things of the world;  it sees only that which pleases its Beloved; it would hear others speak only of its Beloved; every other discourse wearies and troubles it.

    In meditation, the soul retiring to converse alone with God is raised above itself.  "He shall sit solitary and hold his peace," says the prophet (Lt. III,28).  When the soul sits solitary, that is, remains alone in meditation to consider how worthy God is of love, and how great is the love He bears to it, it will there relish the sweetness of God, and fill its mind with holy thoughts.  
(From: Saint Alphonsus; Devout Reflections on Various Spiritual Subjects)