The essential cookie is the PHP session identifier, usually named PHPSESSID. This cookie is required to maintain your login session and ensure a continuous experience as you browse the site. It is automatically deleted when you log out or close your browser (both from your browser and the server).
The other cookie is optional and is used purely for convenience. It stores your email and id so that the login form can be pre-filled when you return to the site. You can safely decline this cookie — you'll just need to enter your email and ID manually each time you log in.