I believe it will be difficult to source a definitive answer to this question of why a CSRF token is "needed" in Magento's add to cart GET action. I'll make an attempt to interpret its purpose. I'm by no means a security expert and this is my interpretation of CSRF in this particular context.
From [owasp.org][1]
Cross-Site Request Forgery (CSRF) is an attack that forces an end user to execute unwanted actions on a web application in which they're currently authenticated. CSRF attacks specifically target state-changing requests, not theft of data, since the attacker has no way to see the response to the forged request.
One example of this attack is embedding a hidden image in an email or an alternate webpage: