The key to the answer has to do with a notion called information equivalence. If the mapping from the base relations to the view relation is reversible, then appropriate updates can be made. This is not always the case, and so most commercial products only support read-only views. It should be noted, however, that these commercial products are much more conservative than they really need to be in their support for updating views. Interestingly, a view mechanism provides more than just a means of conceptually extending the database with derived relations that are of interest.
Views allow different users to have different ways of seeing the same database.
Views also provide a form of security. A user can be allowed access to a view, but not to the base relations upon which it is derived. A mechanism related to a view is called a snapshot. The essential difference is that a snapshot is not virtual, it is maintained in a materialized form. A snapshot is the result of an executed query.