I meant to write that its a better indicator of the freezer's actual temp.
The water is a better indicator of the actual temp inside the chest freezer due to water transferring heat differently than air. Also, when he opens the chest freezer to check on the yeasty beasty, the temperature of the water will not fluctuate as much as the temp of the air inside the chest freezer. This will help to keep the chest freezer from cycling on and reducing the temp more than necessary. The wort and the water glass will be closer in temp than that of the air, the fermenting wort, and the temp probe.