Why was this track added to the fade of "Do It Again"?
Same reason CIFOTM was used in Surf's Up in 1971. Brian didnt think the material would ever see the light of day, but wanted it to be heard. So he used it to patch bare spots in future works where it seemed appropriate. He probably thought the workshop sounds worked well with the idea of the group building themselves up again.
And I don't blame him for doing that at all. Using construction noises and what-not to create a song is probably one of the most inventive musical ideas for its time. People deserved to hear something like that, even if it wasn't on the album it was originally intended for. Just image what all those young teenage girls thought when they first heard it!