These are a new feature that can be purchased from the shop. You use Galaxy Cash to purchase them. These robots are made by FreeJam and likely unlock new weapons and/or items that are not usually available for that tier.
Pre-Built Robots are best for having a new garage slot and for having new cubes. Also they can be good for having a whole new robot to battle. You also get Premium with them ranging from 3 days to 90 days. Another upside it that you get a much better robot to battle with and unlock new items.
*70% off when bought through madorc.comEdit
**Removed Dec. 8th 2014.