Services are the single most important tool available to perform instance consolidation. In this context, a service refers to the name by which a client can connect to the instance, this is configured by the SERVICE_NAMES
instance parameter, and it defaults to DB_UNIQUE_NAME.DB_DOMAIN
if defined.
Connecting through services in an RAC environment is useful to have shifted the service across instances depending on availability and scalability. Using services in a single instance is not frequently seen. Most DBA's configure the default service and even more, there are DBA's who simply ignore this parameter and configure connections to the database by means of TNS entries compatible with Oracle 8.0 using the Oracle SID instead of the Oracle Service.
In single instance environments sessions can be tuned by services. There are other session management tools like Resource Manager, which can provide different resource allocation emphasis based on the service the user defines to connect to...