What was the purpose of the Domesday Book?

The survey's main purpose was to determine what taxes had been owed during the reign of King Edward the Confessor, thereby allowing William to reassert the rights of the Crown and assess where power lay after a wholesale redistribution of land following the Norman Conquest.

Explanation:

Why was it created?

Because he was under threat of invasion, William wanted it written down so he could figure out how much tax he could charge to pay for an army.

the answer is use of support  

keeping a cash reserve because cash flow is the the movement of money through a company and if it just sits there, it wont the movement