Object databases can be useful in highly object-oriented applications. If you build your classes correctly, the class can handle all the details of loading and storing itself in a database. The Person class, for example, provides methods to fetch and save objects representing people. Its methods hide storage details from the main program.
The XmlSerializer class automatically serializes and deserializes objects. It uses reflection information available in VB.NET to discover what the class looks like. That lets you separate your code from the exact class definition. The XmlSerializer automatically handles any changes to your class without forcing you to rewrite your code. By handling the serialization details for you, XmlSerializer lets you focus on writing the main program, not on information storage minutiae.
For more information on using XML in VB.NET, see Rod's book Visual Basic.NET and XML (Wiley, 2002, http://www.vb-helper.com/xml.htm).