If there's a common theme running through the XML references available on the market, it's probably that XML makes you look at information in a different way: as data. The process for determining the structure of your information and the resulting DTD is very similar to the analysis that a developer goes through to design a database. Database designers are not concerned with the actual data values in design; they are interested in the type of information, the hierarchy of that information, and the relationship of the pieces.
A similar approach is taken when designing the structure of XML. The result is a structural format that can be stored very easily in databases. It can be stored as a series of elements rather than as a whole document, and those elements can be extracted and assembled in any order, based on your needs.