Use constraints to improve the clarity, usability and intuitiveness of your design
Constrains, as methods of limiting the actions that can be performed on a system, simplify usability and minimize errors. For example, dimming or hiding options that are not available at a particular time effectively constrains the options that can be selected.
Proper application of constraints in this fashion makes designs easier to use and dramatically reduces the probability of error during interaction. Simply put; constrains simplify usability and minimize errors!