It communicates purpose. It improves productivity. It can make an application more reliable. It can make the application easier to update and maintain. A past-generation user interface may do little to help the user.
Input is often rigidly sequential.
Data validation occurs after input, and the program typically tells the user only when he or she has made a mistake. Accomplishing this modest capability requires a great deal of validation and exception code. It also improves data validation functions while reducing code. For example, radio buttons offer Yes or No, mutually exclusive options, while check boxes offer multiple choice, mutually inclusive options. The program presents only valid choices.