Using correctly the @Scope annotation is Spring 3 web application

I recently readed carefully about the spring mvc 3 beans scope, specifically the web ones(session, request and global session) and i have some doubdts: If i have a controller, why should i annotate him with other scope aside of singleton? I mean, the controllers are supossed to handle the requests and instantiate the view resources of all the app, so why give them a, for instance, session scope? what is the advantage of do that? Is advisable making the services layer session scoped? And finally, is there any convention or good practices that dictates where and when is more convenient the use each one of the web scopes? If there is, can somebody provides me the link or information about it? Not necessary convention or good practices, also your experience about it. Thanks very much.