SCREWDRIVER BOMB

Screw·driv·er /ˈskro͞oˌdrīvər/

noun

1. a tool with a flattened, cross-shaped, or star-shaped tip that fits into the head of a screw to turn it. 

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The main reason of screwdriver bomb shape is to have a good grip to unscrew the tight ones without suffering in the trial. So that´s why screwdriver bomb has a rough  and confortable design to allow the user use all the hand force to unscrew the hardest ones and also to have a quick access to the different bits for every occasion.