Faster build/upgrade time.
I'm not sure if this has already been suggested, but my idea is that you can decrease your build or building-upgrade time by adding a builder to the process.
For example, upgrading to a level 7 town hall takes 6 days. This feature allows you to add one (or more) builders to the upgrade process to speed up the time.
Obviously not cutting the time by half, but maybe by a percentage, and more builders would increase the percentage.
Normal time - 6 days
Builders -
+1=15%
+2=+5%(20% total)
+3=+5%(25% total)
+4=+2.5%(27.5% total)
(just some rough numbers)
So yeah, that's just my suggestion, it's a benefit to those who purchased the builders, and to Supercell.