The original question was to know how Google was driving their G+ feedback feature. In the context of capturing user feedback, the point of a screenshot is to relay information about the state of the user's environment to someone else, without the need for physical proximity.

In my experience, users who report feedback are typically concerned more with the data on the page than how the page itself looks when it's rendered. In these cases, a screenshot acts as proof and can immediately focus the problem without requiring a lot of written description. Among users who have non-trivial feedback about a website, the likelihood of their being able to clearly and concisely describe their problem is vanishingly small, and removing the friction of attaching screenshots to their issue reports is immensely helpful.