The most common reason for making an Act of Spiritual Communion is when a person cannot attend Mass. Acts of Spiritual Communion increase our desire to receive sacramental Communion and help us avoid the sins that would make us unable to receive Holy Communion worthily. 

Prayer for Spiritual Communion

My Jesus, I believe that you are present
in the Most Holy Sacrament.
I love you above all things and I desire
to receive you in my soul.
Since I cannot at this moment receive
you sacramentally, come at least
spiritually into my heart.
I embrace you as if you were already
there and unite myself wholly to you.
Never permit me to be
separated from you.
Amen.