We are to disciple others because this is what Christ did and what Christ has commanded just right before he went up to heaven. In the gospel books, Matthew-John, we can see how Jesus called his disciples, discipled them, and taught them about things that they have to know.
He taught both in academic way through his lectures and parables and he taught them through practical ways through their ministerial experiences like the feeding of the 5,000 people (Matthew 14:21). One of the main foci of Jesus Christ is about faith, and Christ taught his disciples on having faith.
So why disciple? Because it is first done by Jesus Christ. And as his followers, we are to do the same. Secondly, it was commanded by Jesus Christ just before he ascended to heaven. And third, to fulfill his purpose of calling us. Furthermore, this is an act of worship.
To know more about a “Disciple” in Biblical perspective, read “Chrisitan Disciples According To Bible“.