The simple reason is that Jesus gave us the sacraments because He wants us to use them.
To go a little deeper, mortal sin kills the life of Christ in your soul - you are spiritually dead. The Holy Eucharist is the food for the soul, and food cannot nourish a dead person. There are two sacraments that give life to a dead soul, Baptism and Reconciliation (Penance, Confession). Baptism can only be received once. For mortal sins committed after Baptism, the only way to revive the soul so it can benefit from the other sacraments is to go to Confession.
Someone will surely provide chapter and verse for St. Paul’s warning that to receive Communion unworthily is to eat and drink condemnation upon oneself.
People who ask questions such as yours are often asking because they are hesitant to go to Confession for some reason. They are sometimes afraid that telling their sins to the priest will cause the priest to think less of them, that the priest will be shocked and yell at them, or some other negative thing. I assure you, as one who has confessed some really shameful things, that the priest (1) has heard everything and will not be shocked, (2) only wants to encourage you and welcome you back and will not yell at you, (3) is bound by the seal of confession not to mention your sins to anyone, ever, including yourself, and (4) promptly forgets what sins he has heard.
Additionally, the graces you receive from confessing and being forgiven are amazing. You are restored to the friendship of God, with no doubts at all, such as you might have if you tried to be sorry enough by yourself. In order to have mortal sins forgiven before confessing them, you must have
perfect contrition, that is, sorrow because you have offended the good God, not for any human motive, such as fear of hell. In confession,
imperfect contrition is quite enough. If your contrition refers to God in any way at all, including fear, that’s good enough for absolution to work. Also, you receive specific help (grace) to conquer the sins you confess by name. What could be better?
It’s Saturday - why not go to Confession today? Here’s a nice
Examination of Conscience to help you out.
Betsy