I'm not a historian but you don't have to be one to know this. When you write out the amount of money, you include a decimal point to show that the numbers after that point are fractions of a dollar/euro/whatever.

The point of putting the currency symbol before the dollars is so that a forger/theif/dishonest person can't just write a 1 or a 10 in front of the amount (I want to write a cheque for 10.50$, my sneaky grandmother changes it to 110.50$)

Putting the currency symbol before the dollars stops this. It's that simple.