Hi Kiran,
We use AET when you want that data to be available in all its related database tables and field will be appended automatically in related structures/BAPI structures etc. In this case user can enter data and save in database. You can do many things with AET. Adding custom fields using Application Enhancement Tool (AET)
In your case you need two fields only for display purpose so we go for value attributes.
Consider below example :
You have the field in UI called Birth Date ( AET field or standard available field ). Now you want to calculate Age of a person. You don't need to create AET field for Age. Create a Value attribute for Age and write your logic in its getter method to calculate age based on current date and birth date
( Current date - birth date ).
Regards,
Pallavi