Domains actually are data types with optional built-in constraints. The main advantage of using domains is to have in one place a data type with a specific validation instead of creating separated columns in tables and applying to them the needed constraints to provide correctness when assigning values.

One simple example is an email column in different tables: the data type, semantics and the validation for this column are the same, thus a neat approach is to create a domain called email with the needed constraints and assign it as the data type of the columns which requires it. This is the principle of object reuse: no redundancy or repeatedly create the same object with the same purpose. Additionally, you have only one object to do the changes when needed.