When to use @Transactional annotation in unit testing?

Question

i have a junit test class which its configuration is as follows:

@RunWith(SpringJUnit4ClassRunner.class)
@TestExecutionListeners({ WebContextTestExecutionListener.class,
        DependencyInjectionTestExecutionListener.class,
        DirtiesContextTestExecutionListener.class,
        TransactionalTestExecutionListener.class })
@ActiveProfiles("test")
@DirtiesContext
@ContextConfiguration(loader = AnnotationConfigContextLoader.class, classes = {
        SpringConfig.class, SpringTestingConfig.class,
        SpringLocalContainerJPAConfig.class, CustomConfiguration.class })

and the test methods is calling service methods and the service class has the annotation @Transactional for some reason, when i make test case for service method it fails and give me the exception org.hibernate.PersistentObjectException: detached entity passed to persist as in this post: org.hibernate.PersistentObjectException: detached entity passed to persist with H2 in memory database and to fix this exception i have to annotate the test class with the annotation @Transactional although in the managed bean when i invoke same code it works fine, so my question is why @Transactional is required sometimes in test and when i should annotate my test class with @Transactional considering that service layer has this annotation.