Effortless Building gives you the ability to mirror your construction and destruction of bricks in a designated set of coordinates. So you can double or quadruple your efforts in whatever task you need.
So if you have some repetitious building to do, this mod can cut your time down immensely. If you have some similar looking walls to put up, now you can do double the work in half the time. And if you have some big holes to dig then destruction of blocks works with this mod as well.
The possibilities are pretty great not just for building or breaking but also for mining and farming since making trenches and veins are a snap with this mod.