This is a display base or stand.
From the Kotobukiya website:
Kotobukiya’s Mechanical Chain Bases offer mecha modelers and mecha action figure collectors alike the perfect hangar bases for displaying their machines in development, repair, or storage. They come molded in dark gray with lots of accessories, and each base can be assembled easily within minutes. Fully compatible with Frame Arms, Frame Arms Girl, M.S.G. lines and more, VER. TYPE-B brings back the previously released 004-006 numbering and comes with 2 wall parts, 2 floor parts, 3 large containers, 3 medium containers, 3 small containers, 1 large shuttle, 1 medium shuttle, 1 small shuttle, lift trucks, container truck, tractor, weapon dolly, projector, projector mount, catapult, and 1 general-purpose consolidated parts set. Multiple bases can be connected to form larger and more complex Mechanical Chain Bases, and a vertical wall part from another set can even be mounted onto the top of the wall to extend it. As a special feature there are multiple 3mm holes in the floor allowing for different placements of the catapult!