The tree method is luxurious in comparison to streams. To use an analogy, think of a stream generator as a hose gushing out XML. A tree is that same XML frozen into an ice sculpture. You can peruse it at your leisure, returning to any point in the document when you need it. This structure requires more resources to build and store, so you will only want to use it when the stream method cannot help.
There are many reasons why a tree structure representing a piece of XML is a handy thing to have. Since a tree is acyclic (it has no circular links), you can use simple traversal methods that won't get stuck in infinite loops. Like a filesystem directory tree, you can represent the location of a node easily in simple shorthand. Like real trees, you can break a piece off and treat it like a smaller tree. Most important, you have all the information in one place for as long as you need it.
This persistence is the key reason for using trees. If you can live with the overhead of memory and time to construct the tree, then you will enjoy luxuries like being able to pull data from anywhere in the document at any point of the processing. With streams, you are forced to work with events as they arrive, perhaps storing bits of data for later use.
Tree processing is usually object-oriented. The data structure representing the document is composed of objects whose methods allow you to traverse in different directions, pull out data, or modify values. DOM, as we will see later in the chapter, is a standard that defines the interfaces of objects used to built document trees. Encapsulating XML data in objects is as natural as using markup, with as many benefits.