It's a great way to explore a site and group related images when it's difficult to type a precise query. The top results provided by the Similar Images feature are much better than the results for [monuments in Paris site:wikipedia.org], where you can find maps, flags, logos.
It's important to note that Google Image Search's site: operator no longer takes into account the URL of the image, so if a blog includes an image from Flickr, you'll still be able to find the image when you restrict the image results to the blog's domain or subdomain. "In the past, the [site:] operator filtered based on the image URL, not based on the URL of web pages linking to the images. Now, the operator will run your search over web sites that include images, no matter where the images themselves are hosted, which removes a lot of noise from your results and gives you more control over what you're searching for."