It allowed people to be quantified with a single number, 'he owes X amount of dollars', 'he has X amount of dollars'.

During the dark ages, and early middle ages, people didn't have much money. Instead there were complex relationships based on obligations 'loans', IOUs, real estate transactions to hide usury, dozens of different currencies with different exchange rates. Determining wealth was a complex affair.

On a completely different tangent, Leonardo da Vinci probably illustrated the first book about double entry book keeping. He was interested in math and knew the author, most likely traded his artistic skills for math lessons.