DOM builds the complete XML tree in memory, which takes time and uses up a bunch of memory.
SAX, on the other hand, does not build any in-memory stuctures, and allows the developer to do something just with those nodes that the application is interested.
You'd use SAX if you don't need the XML tree (say, for later modification and saving), and want to save resources (time, memory). Drawback is (obviously) that you don't have the XML tree, so you can't easily determine, e.g., the parent or sibling of a given node.
Ping & DNS - updated with new look and Ping home screen widget