Here are a few reasons, drawn from Scripture, why Christians should engage in apologetics:-

1. Apologetics is a biblical command, 1 Peter 3:15.
2. Christians are to contend for the faith, Jude 3.
3. Christians are to pull down strongholds that stand against Christ, 2 Corinthians 10:4.
4. Christians are to eliminate doubts in the mind of the convert, 2 Corinthians 10:5.
5. Christians are to give a reason for their hope in Christ, 1 Peter 3:15.
6. False doctrines and ideas should be addressed, Titus 1:9-11.
7. Without evidence it is unreasonable to believe, Luke 24:39; Hebrews 11:6.  God gave evidence of his existence and nature in creation, Romans 1:19-21.
8. Jesus, Paul, and others engaged in apologetics, Matthew 22; Acts 17; 18:24-28.

Based on House & Holden, Charts of Apologetics and Christian Evidences.