iOS ships with a few core apps such as a calculator and a weather app, which are there for your convenience. Yes, you might have found third-party replacements for them, but they're still nice to have when you are in a pinch. And if you spend some time with these apps like Lex Friedman of Macworld did, you will discover they have a few hidden functions that'll surprise you with their utility.
An excellent example is the Calculator app which is a simple calculator in portrait mode, and a scientific calculator in landscape. Similar features exist in the Clock, Weather, Stocks and Voice Memo apps. You can check out all these lesser-known functions in Friedman's Macworld article.