The first parameter is the uid (usageContextId) which is only an URI. The configure method doesn't need to know the context definition but need to know the service configuration for a usage context id.

So in other words, since the same UsageContext could be used to configure several services, it is not useful to add Annotation for every configured services. The idea was to use to separate the UsageContext which refer to a "usage" of the system (think about different user's roles) and the Annotation which refer to the configuration of the service. Thus the UsageContext  is kept very light.