The most likely explanation for this uniquely Colonial American development was that adding a lean-to on the back of the house was an inexpensive way to enlarge the house to accommodate a growing family. Queen Anne's "story tax" may have also had something to do with it - it cost more to have a fully two-storied house than one that was half and half, like a saltbox.
A larger house also meant more room for gatherings of family, friends, and perhaps fellow worshippers in home church meetings. Especially in New England, winters are long and cold meaning people would need indoor spaces in which to gather.
I have made a primitive penny rug or candlemat reflecting these values and developments and listed it on Etsy. You can see it here: https://www.etsy.com/listing/212748402/saltbox-house-penny-rug-wool-blend-felt?. Let me know what you think of it.
