The only two reasons I use my thermometer are to dictate when swim season is over, and when it begins again!! My kids don't understand "we can't swim yet, the water is still too cold", so if I tell them "okay, we'll swim when the thermometer says 80" they don't bug me to death about it.

Since my pool stays open year-round, I use a water temp of 60 in the winter to determine when to do my major shock.

Otherwise, I don't know why you'd necessarily need to know the temp.

Janet