Why Schemas?

Interoperable Descriptions

Common Vocabularies Establishing common vocabularies makes it easy to build software that processes information according to a clearly defined set of rules. The larger the audience using the same vocabulary, the larger the audience.
Formal Sets of Rules Because machines (computers) will be doing most of the XML processing, expressing those vocabularies in a form that computers can understand is important. The formal description must be regular, unambiguous, and relatively easy to process.
Building Contracts On the human side of the information interchange equation, formal descriptions of vocabularies can provide a core set of rules for all participants in a series of transactions. Schemas can make it clear which kinds of information are required or optional for which kinds of transactions.