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Because you plug it into a thermocouple device that you poke inside the middle of your roast, which is a completely different temperature than the air in the grill. This is an addon Bluetooth thermometer Option for Weber grills - I have a Weber but didn’t get this option.

I think the others are missing your point a bit.

I use an IoT thermometer so that I can monitor smoking progress over 13 or so hours and still do things like go to the hardware store.

Placement. Either you want a probe thermometer measuring the internal temperature of the meat or an air temperature probe on the indirect side of grill. The gauge on the grill is likely to be off 50-100 degrees F. They're often cheaply made and not as high quality as something like a probe themometer from Thermoworks, or presumably this Weber device.