You have learned that a schema can be used effectively to ensure that the structure of the data being marked up is what you expect it to be. In particular, with measures in place for validity, you can do the following:
Enforce the use of a predefined set of tags.
Ensure that the elements and their attributes are in the precise order required by your document content or business rules.
Control data types (provided you employ a schema approach that facilitates data type constraints).
Control data integrity for optimal transaction-based exchange of information over the Web.
Enforce all of these rules, controls, and constraints.