We use it to modify the column names, so they show up customer friendly. Another use to make the columns available for selection in the QBE lists for users to pick.
Data Policy produces data integrity by enforcing rules at the table level. It basically acts like a filter on the table layer.
Some Important Features include::
1. Apply default values
2. Set mandatory fields
3. Set read-only field for specific users
4. Set conditions when a field is visible or invisible
5. Moreover, we can apply validation rules as well.
Hope that helps.
Generally we don't use data policy much because changes/condition fixed on it will be applicable to module irrespective of open/update/close state of that module.