Yet Google would have to know what the address numbers really was in order to validate the reCAPTCHA, so that can hardly be why they are doing it. They don't need to crowd source an answer that they already know.
Doubtful. They post two images. One they know and one they don't. They use the data for the one they don't, combine it with data from 1000s of other people who have also solved that captcha to get an accurate picture of what that particular number is. They use the one they know to validate the recaptcha data and verify you're human...
the preceding comment is my own and in no way reflects the opinion of the Joint Chiefs of Staff
was not - it was Jack Valenti's
A list is only as strong as its weakest link. -- Don Knuth