Oracle Network Administration - Oracle 10g Database Administration Concepts and Implementation Made Simple - page 227

6) Just to access Oracle Enterprise Manager Database
Control, you need not to have tnsnames.ora file created
on the machine from where you want to use the Oracle
Enterprise Manager Database Control. Oracle
Enterprise Manager Database Control has a web-
interface and can be accessed from anywhere using any
Internet Browser e.g. Opera, FireFox, Internet Explorer
etc.
7) Then why we need Net Service Name? My answers is,
whenever you want to use SQL*Plus, MS Access etc. to
connect to Oracle Database Server either on the same
machine where you have Oracle Database Server or
from a different machine located in different part of the
world; you need to install Oracle Net and configure it
for client side i.e. to create tnsnames.ora file.