I'd bet 90% of iOS users haven't considered replacing the calculator that comes with their iPhone. After using Calcbot, those 90% should at least be using the free version of Calcbot. The stock calculator only lets you do calculations in a one off fashion. There is no account for keeping a running history, the ability to favorite calculations you use often, any scientific calculations, or the simple clarity provided by seeing the current equation for the answer.
There are many other features that come with the free version. Check out the developer's site to learn more about these.
The place where Calcbot really separates itself from other calculators is with the premium version. For a couple bucks you get the ability to convert over 500 units in 22 categories like temperature, currency, and weight. In addition to these conversions, you'll also have access to custom themes, unlimited history, and the ability to set custom constants.
If you find a calculator on your iPhone useful, $1.99 is a small price. You can't even get a cheap keychain calculator for that cheap.