The purpose of labels is to make it easier to do calculations with Maple. They are especially useful if you are doing calculations that have several steps, with the results of each step being used later in the calculation or for results that will be used in more than one place in subsequent calculations. Used properly, labels help you organize your work, reduce the amount of typing required, and reduce the risk of errors.
Finally, don't forget that you can go back and change a Maple worksheet command. If you define an expression without a label, but decide later on that you should have used a label, just go back and edit the original command to add the label!