Also, if you are asking why someone would use the arrival sensor instead of just Geopresence on their phone, there are two answers to that.
The first is that you can use the fob for someone who doesn’t have a smart phone, or who doesn’t want to install a tracking app on their phone. That might be a child, the dog walker, a cleaning service, an Airbnb guest, a neighbor, in-laws, etc. People that you might give a key to your house, but for whatever reason an app just doesn’t work.
And the second is that different forms of presence detection work better at different specific street locations. Some people find it the fob works better than the phone, some people find the reverse. It just depends on local conditions.