BTW I didn't get what the C# code
generated by "xsd.exe" is worth for.
I am assuming what you mean is "I don't understand how the generated code is useful"
The purpose of the code it generates is to serialize using the Microsoft serialization subsystem in .NET. If you create a new XmlSerializer(typeof(GeneratedType)), you can then call Serialize() and Deserialze() on it to go to/from Xml and objects.
In a more complicated code generator, such as CodeXS, it becomes even easier, as they generate helpers for you: GeneratedType.FromXML(Stream/String) to deserialize and myGeneratedType.Xml to serialize.
These generated classes allow you to work off a published schema, and have total confidence that any XML generated that meets the schema will parse and be generated using these types. You don't need to do any work to get the data out of the XML (ie no XML DOM access) and you don't need to think twice about generating XML that is compliant with your schema. It just works :)