I was reading up on this again and it appears many artisan distillers 1800's) added both backset and then actual fermentation lees to the the next mash, both to save the amount of fresh yeast needed for the next ferment and to improve flavor. You get lactic acid development which (as in some brewing) increases the complexity of the final spirit whether aged or white. One source said using only backset for the next mash was half of the original sourmashing process, i.e., that continued use of lees resulted in the best taste.

Reading all these old techniques, there is no surprise really that modern bourbon seems in general lighter and thinner than it was even in the 1970's.