You can create a Test Job when you want to execute a collection of Test Suites after each other.
The Test Suites can test different instances of the same AUT, or different AUTs entirely.
You do not have to create a Test Job if you only want to run tests on one instance of one AUT. To do this, you can use a Test Suite (Section 3.16, “Working with Test Suites”). A Test Job is most useful when you want to switch between two AUTs (either the same AUT or a different one) in one test run.