Someone asked once about praying prayers someone else wrote. The person very much believed in spontaneous prayers and conversations with the Lord, which is very good.
There are just times when I need the memorized prayers or praying the Divine Office, the Rosary, the Divine Mercy Chaplet and novenas. Reading the Psalms and Holy Scripture strengthens my faith, so my response to the question of why I pray prayers I didn’t write was “Have you ever sung a love song you didn’t write to your spouse?” The words are meaningful, it is something you want to say to your spouse.. For me there are times I need the prayers and words of others.
From morning prayer
[A soul thirsting for God
Whoever has left the darkness of sin yearns for God.]
O God, you are my God, for you I long;
for you my soul is thirsting.
My body pines for you
like a dry, weary land without water.
So I gaze on you in the sanctuary
to see your strength and your glory.
For your love is better than life,
my lips will speak your praise.
So I will bless you all my life,
in your name I will lift up my hands.
My soul shall be filled as with a banquet,
my mouth shall praise you with joy.
On my bed I remember you.
On you I muse through the night
for you have been my help;
in the shadow of your wings I rejoice.
My soul clings to you;
your right hand holds me fast.
Glory to the Father, and to the Son, and to the Holy Spirit:
— as it was in the beginning, is now, and will be for ever. Amen.
Ant. My soul has held fast to you, my God; for your sake I suffered death by stoning.