Some websites share lists of activation codes. These codes are frequently obtained illegally, for instance, from compromised accounts or through credit card fraud. Even if one of these codes works temporarily, it will almost certainly be blacklisted by Avast's servers during a regular license check. Your "premium" protection will then be deactivated, leaving you with a non-functional, or worse, a purposely crippled version of the software.
For Windows users, Microsoft Defender comes pre-installed, updates automatically, and ranks highly in independent antivirus testing laboratory reports. It provides comprehensive, lightweight baseline protection for free. To help you find the best setup, tell me: Some websites share lists of activation codes