Data validation makes sure that users enter only certain values in a cell. The benefit of this is that you can avoid erroneous inputs and it provides significant guidance for users. Good examples of this could be a dropdown menu that only allow you to input months of the year. For instance, if you had to enter your birthdate or if you were booking tickets for a certain date. Other examples could be only using whole numbers, so no decimal places, or you could use a custom list of your own making, maybe only from best, base and worst cases. To help users along, you can design an input message and also an error message if they get something wrong.