Suppose you have two classes that should be registered with Spring context exclusively, e.g. only one of the beans must exist in the context at any time based on some boolean property value. Of course you can do it by adding explicit if condition into your @Configuration class. But if the classes have no common interface it may be quite cumbersome. As an alternative you can use @ConditionalOnProperty annotation on your classes, e.g.:
@Service
@ConditionalOnProperty(name = "use-left-service", havingValue = "true", matchIfMissing = false)
public class LeftService