The primary purpose of a phishing filter in email clients is to protect users from malicious attempts to steal sensitive information, such as usernames, passwords, and financial details. Phishing filters analyze incoming emails for suspicious characteristics, such as known phishing URLs, deceptive sender addresses, and unusual subject lines. When a potential phishing attempt is detected, the filter can warn the user or automatically move the email to a designated folder. This helps prevent security breaches and safeguard user data, contributing to a more secure online communication environment.