The purpose of a web application firewall (WAF) is to protect web applications by filtering and monitoring HTTP traffic between users and the web application. A WAF helps defend against various threats, including SQL injection, cross-site scripting, and other common web application attacks. By placing rules and policies to analyze incoming requests and responses, the WAF can block potentially harmful traffic before it reaches the application. This layer of security is essential for organizations to safeguard sensitive user data and maintain the integrity and availability of their online services.