I've used them for everything. I have several and I've used them to monitor temps in grain bed and in recirculating mash liquor when the temp is being controlled by another device. It's allowed me to maximize the efficiency of my system and fine tune my temp control by having a better understanding of the relationship between my HLT and my recirculating mash.
I've used one for controlling a hot plate when I had a direct-heat mash tun with recirculation and used one to monitor temp of strike water on the stove for mashing in before I had an HLT system.
Definitely used them for freezer control and right now I have one set for Celsius operating my AC glycol cooler setup and another reading in Fahrenheit to monitor the differential in the opposite end of the tank. I use them to control the pumps on my glycol system.
I alsohave one that can be set for several different time and temp changes and can be used for step mashes as well as controlled fermentation temps for lagers that need to be pitched, fermented, D-rested and crashed/lagered at different temps over several weeks (though I haven't really put that one through a lot of testing).
Find them as cheap as possible and buy as many as you can find.