2what it does while you build
It attaches an actuator to every block as you place it, pulling from the actuators sitting in your inventory. A retractable wall becomes one pass instead of two. The toggle sits beside your inventory so you can switch it off the second you go back to placing ordinary blocks, and it works from the inventory without taking an accessory slot.
when it beats the grand design
The Grand Design places actuators more precisely, but only in straight lines. That is the whole difference. For a flat slab or a long wall it is fine. For anything irregular, stairs, zigzags, scattered platforms, going back over the shape a second time with a line tool is slower and easier to get wrong than having the actuators appear as you build. Keep both if you can. They cover different jobs.