In a typical business application, you have screen objects like an entry for the phone number or an OK or Cancel button. Most applications contain more than one form with one of these same objects. Beyond that, many apps use the exact same controls as other apps. So essentially you're duplicating work over and over. Now suppose you want to make a change to the standard, like the phone number format mask or validation, or even the color of a button or entry textbox.
If you make a change to your single class definition, changes will propagate throughout every form, application, etc. which use that class. You could change the standard color of a particular control and you wouldn't even have to open any of the existing forms in order to inherit the change. Just run the form or program and the new properties and methods take effect.