Followers of Jesus Christ need to be discipled and trained.
All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be complete, equipped for every good work. 2 Timothy 3:16 (ESV)
The LORD Jesus Christ selected His disciples and then spent time with them imparting knowledge through His teachings and examples thereby preparing them for the years ahead.
Athletes, soldiers - these are some of the metaphors used for believers in the the LORD Jesus. As such, we need to be equipped for the battle that wages war against us.
Through discipleship, believers are trained to be well-grounded in solid Bible-based doctrine that strengthens their faith and helps them to stand in the face of opposition, testing and trials.