Explaining why did darius i create satrapies

Darius created Satrapies because the Persian Empire needed a system of government/rulership, but it's sheer size meant that he could not control it centrally. He thus devolved authority to the satraps so that they could effectively manage their domains and ensure that the individual parts of the empire could function properly. 

Could you give me some more details so i can you?

Bis the answer because it deals with social insurance.