Be it for double-opt-in compliancy or a new registered user for your app or website, validating an e-mail address is something you have to do. And it's not only you, it's what other hundreds of services are also requiring their users. 

This takes time, and if anything Google thought us is that faster is better. By using image requests you can take a step away from the process and let users validate their e-mail straight from their inbox. 

Later edit: Following the feedback here and HN a solution for mail clients that automatically load or prefetch images is to disable this kind of validation for them. 

It's also worh mentioning that Movable Ink offers some very interesting uses for dynamic image requests.