When you first heat sugar solution, it boils at a temperature specific to that particular sugar-water ratio. As the water boils out, the solution gets more concentrated and the boiling point goes up. Eventually, all the water boils out and the temperature stays at the boiling point for sugar alone. A syrup with more sugar and less water makes harder candy and also boils at a higher temperature. As you boil the syrup you know water is disappearing because the boiling point goes up. When the boiling point reaches the right temperature, you know you have just the right ratio of sugar to water. That way, when the mixture cools you’ll get the kind of candy you want.