In a real world web application, it's common to collect data from database, and show it in
tree. The underlying data may be hundreds of thousands records. It's impractical to load them all to the client. Therefore ZK introduces concepts of
Paging. To separate view from data, ZK also introduces the concept of
model. It is easier to use different views to show the same set of data. To show data in the view any way you like, a developer may implement his own renderer; ZK has provided a default renderer.