Pardon me if this is stating the obvious, but the captcha images being decoded serve a second purpose. In short, Goggle is crowdsourcing the decoding of those images in a mechanical Turk sorta way.
Why street signs and house numbers? Because when your taken as much Street level photos as they have some are gonna need human review.
At first, ReCaptcha was used to transcribe books, and then in 2012 Google began to use it for street data. The GP is referring to its newest use, which is based on an image classification algorithm. Image search used to rely on nearby words on websites and in the URL, since computer vision was just a research technology.
Why street signs and house numbers? Because when your taken as much Street level photos as they have some are gonna need human review.