Tnsnames.ora is found on client and server sides. Listener.ora is only on the server side. So if you wanted to connect to the db, Oracle looks for tnsnames.ora on the client side and then tries to match it to the listener.ora on the server side.
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Tnsnames.ora contains the name of the db, port# and hostname/IP of te server. Listener.ora contains the same info. When these match, a connection is established.