@Autowired is the annotation used for indicating that you want an instance variable (or setter for one) or a constructor to be autowired using the beans that are available within the application context.
If you want to initialise the beans using annotations you have to mark the class with one of Spring's stereotype annotations:- @Component, @Repository, @Service, or @Controller and also remember to put <context:component-scan base-package="org.example"/> into your applicationContext.xml file so that these annotations are looked for.
More information can be found here spring docs.
Why you would do this is down to what is prefered.
Do you want lots of xml files to maintain? Or do you want to maintain the classes? Annotations sometimes make it harder to know what's going on due to the "magic" that tends to go on in the background rather than you having to explicitly create all your beans.
Personally I'm in favour.
I love this place!
subject: With or without bean definition in applicationContext.xml