When Intergraph first developed applications that used databases, those applications were hard wired to a particular database type. To support the use of a different database type, another version of the application software would have to be written. A methodology was required such that a single version of an application could interface with multiple database types---the solution to this problem is RIS.
RIS decouples the database from the application. By doing so it allows multiple client nodes to share a single copy of the database. RIS includes the networking software to access the remote database, saving you money by not requiring the use of the database vendor's networking software.