Dummy fields in odoo.

Dummy fields is inherited from function class, that means it is like search functional field.
when you use dummy field no value is stored in database.
Dummy fields can be used for backward compatibility of certain field.
Suppose you don't need phone field in customer model anymore, but some other modules still have a reference to this field. So, instead of deleting field, you can replace it by a dummy field.

In Odoo Dummy field always return Null value.