Ritty grew tired of having his profits stolen by less-than-scrupulous bartenders. He invented the register as part of an effort to track his funds and keep track of how much money he made.
They come with a series of buttons that allow for a cashier to input the sales of a product, ring up a sale, and then give change. They usually come with some form of integration with credit card payments, meaning that a credit card purchase can be easily rung up.
The cash register and its successor, a Point of Sale system, are no longer the domain of one exclusive company. They can also be purchased from almost any major website, including Amazon, Office Depot, and Staples.