You also asked how people are using these in your title. I have 9 of them, and so far they are deployed like so:
Garage Water Main
Was afraid the valve might start dripping.
Water Heater (body)/Garage Drain (probe)
Have had a water heater die by leak twice, and I was curious is the garage drain was ever used (which could also be an indication of a leak).
Washing Machine (body)/Front Faucet (probe)
Had washers hoses break before and spray water all over the room. Now THAT is an emergency, lol. Also, this is right next to the front yard faucet, which can break the internal wall hose connecting to it if you have something connected to the faucet outdoors over the winter. So the probe is actually behind the dryer, which is only separated from the front faucet by a drywall.
My Toilet
Had some seals start leaking before. Also a toilet feeder valve there.
My Bath R Sink (main)/Rear Faucet (probe)
This is right next to the rear patio faucet, which can break the internal wall hose connecting to it if you have something connected to the faucet over the winter.
Main Toilet
To monitor the seals and feeder valve.
Main Bath L Sink
This is the sink that gets used the most.
Kitchen Sink
Garbage disposal seals, etc.
Refrigerator
Had 2-gallon water bottles spring leaks before.
I put all of these in a group called “Leak Sensor Group” to cut down on clutter, and created rules to sound the clerestory window V3 siren if a leak is detected by any of them. Also, it sounds the siren a second time if the problem hasn’t been addressed in 10 minutes.
I wish I could apply the rules to the group, would have saved me a lot of work! I would have said if ANY leak detector in the group senses a leak, sound the siren. 
I still am considering:
- Dishwasher (not in use ATM because seals leak, lol)
- My Bath L Sink (not used as much as the R Sink)
- Main Bath R Sink (not used as much as the L Sink)