He didn’t like attics, and hated the small bedrooms that hid behind dormers so he removed them. His clients were mostly the affluent, so this is where the household help (maids, butlers, cooks, etc.) resided. By removing that space, the help had to have rooms in the main portion of the house. This made them more “equal”.
In this case, I think Wright’s motivation was purely architectural, even if it did have some social impact. However, it does illustrate the power that we have at times. That even when being hired for something like designing a house, we are actually given the opportunity to change people and their beliefs perhaps utilizing different motivations than expected. In this case, Wright wasn’t saying, “You should give your help better rooms.” He was saying, “I can make you a better looking house if we don’t do things this way. The help will live in the main floor.” The impact on the help was the same.
This is a clear look at motivations, and that sometimes to get what you want, you may need to find an alternate motivation for the person you’re seeking to change.