A set of OWL statements by itself (and the OWL spec) can allow you to conclude another OWL statement whereas a set of XML statements, by itself (and the XML spec) does not allow you to conclude any other XML statements. To employ XML to generate new data, you need knowledge embedded in some procedural code somewhere, rather than explicitly stated, as in OWL.
For example, "Parenthood is a more general relationship than motherhood." and "Mary is the mother of Bill" together allow a system conforming to OWL to conclude that "Mary is the parent of Bill". In this way, if a user poses a query to a OWL search system such as "Who are Bill's parents?", the system can response that Mary is one of Bill's parents, even though that fact is not stated anywhere, but can only be derived by a OWL application.
based on the logical definition of "subProperty" as given in the OWL spec. The same information stated in XML does not allow you to assert the third fact. XML itself provides no semantics for its tags. One might create a program that assigns similar semantics to a "subProperty" tag, but since that semantics isn't part of the XML spec, applications could be written which conform to the XML spec, and yet do not make that assertion.
Other web languages such as RDFS go a step further than XML, and could support the example just given, but OWL offers a host of other standard properties such as equivalence ("childOf" on an English geneology site is the same as "enfantDe" on a French site), or that particular properties are unique (a social security number is associated with only one individual).
It should also be easy to see that there is similar utility in just about every domain: in finance where one might query about all bank accounts associated with a particular person (whether they are directly owned by, or held in trust for etc), in logistics where one wants to ask the rates for shipping to any eastern European city (where no such category has been predefined and only the countries in eastern Europe are listed). Having knowledge that can be dynamically applied to find an answer, rather than predefined procedures, is extremely powerful.
OWL provides a basic infrastructure that allows a machine to make the same sorts of simple inferences that human beings do.