In event-driven programs information is handled directly. For instance if you want to check employee details, you display a list of employees, select the person and view the details. If you want to make any changes, you can do them there and then. The changes are written to the database when you click a button which has an event routine containing an UPDATE statement. Modes are not needed.
In an RDML function you can display the employee details in the same way, but if you want to make any changes, you have to first click a Change button or enter an option number to bring up a screen with input capable fields and to set the mode to *CHANGE in the program. In other words you signal what action you are about to perform before you actually perform it.
Modes in RDML control whether user input is allowed. In Visual LANSA this is handled using the Read-Only property (True/False) of the component.