What is the purpose of a thermometer on a boil kettle? Never had one myself. If you have one how do you use it?
I only put one in at the end to monitor the chilling. Beyond that, why bother. You can see that it is boiling, and the difference between boiling over and gentle simmer is hard to see on the (typical) thermometer.
It lets me know when I’m getting close to a boil, so that I can watch for a boil over.
Both of the above. Could I live without? Sure. Would I want to live without? No
I did at one point have an a thermometer alarm (like a meat thermometer for the oven). I could set it to 205 degrees F and it would start beeping. That way I could ignore it until right before the boil happened.
Alas, that broke and I never replaced it.
Lets me know chilling temps. I can stop at a temp, 185, 170 F, or whatever to whirlpool. Let’s me know I am at a temp to pump to the fermenter.
^^^this, plus I whirlpool through my CFC to the BK. I can watch my full volume temp.
+3 - also, I have used my BK to heat mash water temps many times. That said, you certainly don’t need one.
Forgot to mention this. I also bring my treated strike up to temp in my BK. I have my HLT filled with RO. Not needed, but for me recommended.
I just got a new boil kettle with a thermometer in December, and so I’ll join the chorus in saying that, while not essential, it sure does make it easier to manage heating and cooling. I can more easily monitor where things are while I’m taking care of other tasks.
The purpose of a thermometer in a boil kettle is to snag on your BIAB bag and put a hole in it.
:o :-\ That’s too funny!
I wish I had a kettle thermo - I use a hand held to find for steeping temps so I don’t evaporate volatile hop oils I’m looking to keep for aroma and flavor.
I think it’s there to impede a hand-stirred whirlpool. Jeez, what a constant thorn in my side! Almost every stir - whack! When you’re juggling an immersion chiller and a hop spider, plus the thermometer probe, it gets to be a hairy proposition. But, as mentioned, it is really nice to keep a tight handle on things. Do I have time to run down to the basement and grab… How many degrees will it drop if I stir 37 times (my age)? You know, hours of fun watching that needle creep down to 65.
It sure beats a floating thermometer - break one of those in a chilled wort whirlpool and you won’t think twice about getting a Thermapen or mounting a hermometer on the kettle! I still can feel the pain from that happening to me years ago!!
Yeah, I think a Thermopen is what I need for Father’s Day this year.
During the boil it’s good to maintain a steady boiling temp for two reasons: 1. Higher temp means you’ll boil off your wort faster. If you know how much your vessel boils off per hour at a particular temp you can better predict how much wort in volume needs to go into the kettle for a desired final volume. 2. Hop isoprenoid conversion is dependent on temperature, among other factors. http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2010.tb00783.x/pdf and so if you want a consistent hop profile across your batches, you should keep an eye on your boil temp. A hop addition at 30 minutes at a higher boil temp might as well been a 45 min addition at a lower boil temp, and this can be important for the resulting flavor and aroma of your beer.
Temperature will never rise above 212 in and open pot… it’s pointless
It doesn’t get much above 210F where I live. 
Thanks, Jeff, for the link to the charts. At my altitude its 210 plus a little smidgeon, which ties very clearly to the charts. If I set my boil controller to 212, it boils over for sure.
Bear in mind that wort will boil about 1-2°C hotter than water depending on gravity.