Why is simply counting currency an inadequate measure of money?

Depreciation on the company's equipment for 2017 is computed to be $18,000.the prepaid insurance account had a $6,000 debit balance at december 31, 2017, before adjusting for the costs of any expired coverage. an analysis of the company's insurance policies showed that $1,100 of unexpired insurance coverage remains.the office supplies account had a $700 debit balance on december 31, 2016; and $3,480 of office supplies were purchased during the year. the december 31, 2017, physical count showed $300 of supplies available.two-thirds of the work related to $15,000 of cash received in advance was performed this period.the prepaid insurance account had a $6,800 debit balance at december 31, 2017, before adjusting for the costs of any expired coverage. an analysis of insurance policies showed that $5,800 of coverage had expired.wage expenses of $3,200 have been incurred but are not paid as of december 31, 2017.