the idea behind RDBMS is to store data once; with minimum space used, maximum speed to find data.

Thus the idea of a parent-child relationship. An object is broken into two parts, the reused part (the parent) and the specific part (the child). Say a customer and their address(es).

XML is a way of storing/describing a data object. It can support a parent-child relationship within itself, for instance a customer with a listing of say 0+ addresses. However, it the paradigm dictates that the object itself should be complete.