The main purpose of a session cookie in web development is to maintain a user's state and track their session across multiple requests. Session cookies store temporary information such as user preferences, authentication tokens, and shopping cart contents while a user navigates a website. Once the user closes their browser, session cookies are typically deleted, ensuring that sensitive data is not persistently stored. This enhances security while allowing for a seamless user experience. Proper implementation of session cookies is critical for functionality in various web applications, including e-commerce sites and user login systems.