Why?
This isn't my idea btw. The Paulian types who want the Fed eliminated favor it.
I think they want gold don't they?
But to answer the question. banks create more money (or credit) because they end up loaning out more than they have. This enables them to pay for the running of the business and pay dividends etc. If they have 100% reserves then they can't create anywhere near as much credit. They can't loan as much out.
So, either interest rates go up to provide enough profit, or they charge you a fee to "mind" your money.
Presently they don't have to charge a fee because they loan out, more than you give them , so they make their interest margin "many times over" on the "same money", at the same time. They multiply the amount of money as it's get re deposited and re loaned, each time putting a fraction in reserve.
This all works fine while the economy grows.