Our God is a holy God and He commands every believer to be perfect in His sight (Matthew 5:48; 1Peter 1:15).
God desires to use believers as His chosen vessels to reach out to the numerous sinners out there (2Timothy 2:21; Matthew 5:16). Holiness is necessary for believers to do exploit (1Peter 2:12).
Sinners cannot fellowship with a holy God because two individuals cannot walk together unless they agree (Amos 3:3).
The believer must be holy in order to see the Lord on the last day (Hebrews 12:14).