They can be used to add information to topic maps if they exist, or also to qualify some information objects even when no topic map is applied. They can be used for example to extract the portions which are in a given language in a multilingual environment. They can also be used to extract only portions of information appropriate to a given security level. Another use of facets is to extract from an information base what is relevant to a targeted audience. There is a certain overlap between facets and scopes.
Facets represent the simple way to implement a mechanism to sort out information.
Scopes are more specific, as they are used to enrich the value of topic maps by enhancing the relevance of the characteristics that a topic map can return. Deciding to use one rather than the other will depend both on the topic map design decisions and on the features offered by the topic map aware software applications.