Last one today.
A new Zone buff, the excited builder, perhaps wanting to get home early to watch the football, would halve build times whilst active. My personal motivation behind this is I sometimes only have a limited time in a morning to get 3 or four copper mines replaced and 4 or 5 iron mines plus an occasional wheat field. I have prem for the additional build slot but only really need faster build times on the occasional mornings I have to go somewhere.