This is super useful feature for debugging. Happens sometimes that something goes wrong and an ad is regenerated after being blocked. So the ad blocker would block it again, thus generating an infinite loop that can take up all the memory (if the ad blocker saves any information about the blocked ads on the current page in memory) and crash your machine in less than a minute in the worst case. In the best case it would just lock up your browser window and you wouldn't know why. Knowing the number of ads blocked would quickly help debug such situations.