Even if Gmail loads tells the marketer that you have opened the message (as people are pointing out, it's a bit unclear), there are still some advantages.
It protects you from the guys at marketer tracking your user-agent, your ip address (which gives a rough geo-ip), the number of times you opened the email, etc. It's unclear to me whether the images could set cookies before (they probably did), but even without that, they could just use etag-tricks, or stuff like that, to track you cross-sites.
Marketers might now know when you open the message, but proxying the image prevents them from getting more precise information.
(No idea exactly how much of this and more Google does, obviously, but they put themselves in position to do it)
It protects you from the guys at marketer tracking your user-agent, your ip address (which gives a rough geo-ip), the number of times you opened the email, etc. It's unclear to me whether the images could set cookies before (they probably did), but even without that, they could just use etag-tricks, or stuff like that, to track you cross-sites.
Marketers might now know when you open the message, but proxying the image prevents them from getting more precise information.
(No idea exactly how much of this and more Google does, obviously, but they put themselves in position to do it)