So they can use it on the go? Most exchanges are designed for desktop view and it becomes a bit messy when using from a phone. So they make a dedicated mobile application so users can easily access the exchange while they are travelling or is away from their computer. Most of these applications also have some sort of push up notification system if there is a significant change in price of a coin (at least coinbase has the function).
Hope you understand now. Just use a little bit of your brain.