Why would you need to put a domain name when install a Oracle database? When you install an Oracle database or instance, you are able to assign a domain name. 

An Example:

SID: orcl

Domain: world.com

Service: orcl.world.com


Why would it be necessary to add or configure a domain?
It allows you to have more than one DB with the same name in the same network. If you have only one it can be the network domain.