Are you asking about Data element with Domain and without Domain ?
With Domain - Offers many advantage like Value tables(This intern helps in defining foreign keys ), F4 Helps, Conversion routines, lower case and sign. And Also future enhancement will be easy with Domain. If requirement comes to increase the length of field than we can easily achieve by changing domain instead of changing all the related data elements.
Without Domain - In case of defining interfaces structure of FM , Methods you can go with this approach.