Answer:
In this case, money is being used as a medium of exchange.
Brooke accepts to be paid in money a quantity that is more-or-less proportional to the amount of work she performed, because she knows that later on, she can exchange that money for other goods and services.
To Brooke, the money in itself would be worthless if she could not exchange for other things, and it would be likely that she would demand to be paid in some other way under this scenario. This is why she is highlighting the importance of money as medium of exchange above the other two functions of money.
