Dr. William Lane Craig's book: "Reasonable Faith" spells out four main purposes for Christian apologetics. I will list what they are and then offer a brief exposition of each:
a). Express our cognitive love to God (Mt 22:37).
We should not only love Jesus with all our heart, but also our minds. In as much as the spirit of a man is instantly converted at saving faith, the mind ought to be the subject of progressive, ongoing transformation as the Christian grows in their faith. Both heart and mind are essential in the cultivation of Christian spirituality.
b). Explain/defend Christianity to unbelievers
This second purpose reveals how apologetics functions as a form of "pre-evangelism". Apologetics ought to lead us to a deeper burden for sharing the Gospel. Often we must knock down the intellectual barriers and opposing arguments to get to the root of a person's rejection of Jesus Christ. The Bible uniformly describes mankind's rebellion against God as moral and spiritual. Apologetics aims to eventually expose this root while also addressing the intellectual branches that issue forth from the trunk of unbelief. The Apostle Paul reminds us in 2 Corinthians 10:4-5 that we aim to bring every thought captive to the obedience of Christ.
c). Confirm our faith to Christians
I have found that Christian apologetics can strengthen the Christian's faith. There are those seasons where all Christians have doubts or struggle in the midst of great difficulties. Christian apologetics can function alongside the internal testimony of the Holy Spirit in verifying to the Christian that following Jesus is worth it all.
d). Explore connections to Christian doctrines and other truths