Topscore, DAT achiever and most kaplan stuff is online. Don't bother taking paper tests unless you have exhausted every possible electronic practice test.
Most (TS, Achiever and Crack the DAT) are on the computer. The only one completely in paper form is DD, like TheWiredNerv mentioned. Even Kaplan comes with online stuff and CD.
Because it is questions designed to test you know the concepts in the question stem and every concept in the choices. You can use it as a guide to study topics you might not have studied otherwise.
It's good for survey of natural sciences and math.