The extra Hanukkah candle

Hanukkah time! Now that it gets dark around 5pm, it feels like the holiday is just around the corner, even though it's over six weeks away. But if you learn Daf Yomi, it's already here! It starts on Shabbat 21!

This post is based on shi'urim that Rav Reuven Taragin gave at Camp Moshava IO during the summer of 2010.

Rava on 21b says that you need "another lamp" in addition to the Hanukkah nerot:

אמר רבא: צריך נר אחרת להשתמש לאורה.

The continuation (by Rava himself?) discusses exceptions:

ואי איכא מדורה - לא צריך, ואי אדם חשוב הוא, אף על גב דאיכא מדורה - צריך נר אחרת.

What is the purpose of this ner aheret?