Most people use it to have events occurring when they arrive home. For example, you might want to have the garage door open, or the porch light come on if it’s after dark, or The television start playing a news channel…
Or you might want to have events occur when you leave home, such as the thermostat adjust lower.
Different people use it for different things and you may not have any uses for it yourself. But if there is something regarding devices controlled by your home automation system that you do pretty much the same way every time you arrive home, then that could be triggered by the presence sensor.
If you are used to using “modes,“ then it is also common to have your arrival change the mode from “away“ to “home.”
We do need to note that a lot of things changed between the classic app and the new “smartthings ( Samsung connect)” app. You used to be able to have your arriving home unlock the door and disarm the security system, but those options are intentionally left out of the new app. You can still do them with a classic app, but since the classic app is going to go away eventually, you may not to implement those options.