When washing towels it's best to wash the towels in the hot water setting of your washing machine which is around 104 F which is what the hot water temperature should be coming into your washing machine at.
You don't have to wash towels in hot water but if someone has been ill and have used the towels then the hot water will kill any germs that may be on the towels.
I just wash my towels in cold water though unless I've been sick and then I set the temperature to hot water to help kill the germs.