Encryption is an evasion technique that is a function of ransomware, which is a type of malware that encrypts the victim's files or system and demands a ransom for the decryption key. Encryption is used by ransomware to prevent the victim from accessing their data and to avoid detection by antivirus or other security tools.
Encryption can also be used by other types of malware to hide their communication, configuration, or payload from analysis. References:
Understanding Cisco Cybersecurity Operations Fundamentals (CBROPS) v1.0, Module 3: Network Intrusion Analysis, Lesson 3.4: Malware Cisco Certified CyberOps Associate Overview, Exam Topics, 3.4 Compare and contrast types of malware