Imagine that you come with a lot of bags from the supermarket and do not have a free hand to press the button of the elevator, or you just rush to the restroom, but it will take a minute for the elevator to come. Or maybe you are at home and you just don't want to wait for the elevator to comes and you prefer it would wait for you.
This is exactly why you need an app that invites the elevator, the app can act if you press a button or it will recognize that you approach the building and invite the elevator automatically.
The app can be for new elevators, but you can connect a device that will be connected to the elevator button and click through the app or it can be manually click on it if you do not use the app.
By the way, a remote control can be used instead of app.