The final of the three-fold role of Priest/Presbyters is as Teacher (if you prefer to have 3 "P"s instead, you could label this role as "Preacher").
Christian Formation is how we learn and grow in our relationship with God. Priests have a specific task in Formation and Teaching because they are set aside and trained to learn the knowledge passed down to us by God through the church. Gaining that knowledge doesn't stop with that training, though. In the Examination for Ordination, Priests take the vow to be:
diligent in the reading and study of the Holy Scriptures, and in seeking the knowledge of such things as may make you a stronger and more able minister of Christ.
Priests are also called to "pattern [their] live[s]" around the teachings of God in our Lord Jesus Christ. In other words, Priests are to make God known through teachings in both word and deed. This is summed up in the Bishop's opening remark during the Examination:
As a priest, it will be your task to proclaim by word and deed the Gospel of Jesus Christ, and to fashion your life in accordance with its precepts.
By proclaiming the Gospel, we make Christ Jesus known to all in the world, and in so doing we bring all closer to Him. That is the mission of all in the church, although Priests, again, have a specific role and calling to that mission.
It is also worth noting that part of the role of Priest as Teacher comes in the form of worship. This is seen in the role of preaching, but it can also be witnessed in leading the Liturgy of the Word in worship.
Teaching is important to the mission of the church. That is why it is important to the role of Priest as Teacher too.