Google Reverse Image Search (Capture) provides two methods for reverse image look up
1. Context menu image search over images
2. Context menu image search over non-image elements
The first option is usually provided by other extension. But the second one is more useful as it is not easy to find image links easily in the most situations. This option allow you to capture an arbitrary area of a webpage and send it to image.google.com for reverse image searching.
For more info about how the extension works please check http://mybrowseraddon.com/image-search.html