The clamp holds the thermometer off the bottom and in the middle of the solution which gives you a more accurate reading.
This clamp works great! I recently ordered one and it was awesome! it held my thermometer well and at any height I wanted it to, so you don't even have to unscrew/screw any clamps when you need to change the size of your beaker/flask!
I own several of these clamps and find that they work fine for holding thermometers, but in reality I just hold the thermometer myself usually. I now tend to use a digital thermometer and just use this or my burette clamp to support the thermometer. If I started over, I'd probably not buy this even though the clamp works as it should.