Configuring multiple domains enables easier user management. When it comes to help desk delegation it is quite cumbersome to create roles and select admins. O365 Manager Plus supports domain based delegation which makes help desk delegation easier and more secure in a multi-domain environment. By delegating domain based roles, the technician would have access to data corresponding a particular domain alone.
As a prerequisite you need to create a technician with the roles you wish to delegate.
To know more about O365 Manager Plus' help desk delegation feature, click here.