What do you guys do with yours? how do you use it? I’ve just had it sat in the box because I’ve got no idea what I can actually use this for, so I guess I’m looking for suggestions.
I initially thought it was something that would detect entry into a room but I’ve since discovered that isn’t the case, and I can’t think of what else I could use it for. I know I could just ignore it but it would be a bit of a waste I guess…
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.
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.
Well that’s the thing, I don’t really know! 
I have a Logitech harmony hub that has certain channels programmed in as favourites and we have the same thing on when we come in every evening, is it possible to tie that in for when I come home? I haven’t really looked at bringing the harmony into play to be honest.
Yes, you can do that if you are using the classic App. I’m not sure if you can do it in the new app, though.
I’ve managed to connect my harmony account to the smartthings account, now I’m just trying to figure out how to get it to play a certain channel
You can create an activity in the Logitech Harmony app which ST can import using the Logitech Harmony (Connect) smartapp. You can create activities to change the channels, not the most efficient method.
I used to AirBNB out one of the rooms in my house and would use the presence sensor (attached to the guest keys) to turn off the AC when guests left. I also had it so if I left, it set itself to a mode called guests only where all the sensors in my room triggered notifications to my phone and triggered a siren. If guests left while in guests only, it would set to full away and all sensors in the apartment would trigger notifications.
I use mine to turn everything off when everyone is gone, start my roombas to clean while I’m out, set my nest to away mode and lock my door. When I arrive lights and appliances turn back on, door unlocks, nest makes my home comfortable, and roombas dock. Basically I use them to make my home as efficient as possible when empty and comfortable when occupied.