Oracle - Why create several service_name in service_names parameter? SERVICE_NAMES specifies one or more names by which clients can connect
  to the instance. The instance registers its service names with the
  listener. When a client requests a service, the listener determines
  which instances offer the requested service and routes the client to
  the appropriate instance.


What is the reason for I modify the parameter service_names like this

Service_names = 'service_one','service_two'


Rather than 

Service_names = 'service_one'
Because in the same database you have multiple applications, multiple workloads,... and services help to:
 - identify sessions when troubleshooting performance issue
 - manage resources for those with higher or lower priority
 - balance between several nodes
 - offload some read-only workloads to a standby database
...