The Noam Elimelech (parashath KiTavo) explains that the Tzaddik is full of all manner of good middot and these good middot get shared among all those who connect to that Tzaddik. The Tzaddik, he says, is called the "western candle" because the Menorah was lit from west to east, the light would spread through all the candles from west to east. The Tzaddik is the western candle because the light of HaShem spreads from the Tzaddik to all of the Jews.
So then why do we learn in Yoma that a Yisrael could light the menorah? Not to tell us just that were a Yisrael to light the menorah it would be kosher because in practice that never happened, but more to tell us that a Yisrael can be a Tzaddik too. Even though the Kohanim get to serve HaShem all day long in the Beit HaMikdash, a Yisrael who lives and works and eats outside of the Beit HaMikdash, in the world at large, can still develop a tremendous closeness with HaShem. For, as we know, HaShem fills the whole world. (ממלא כל העולם כולו)
Why do we learn this lesson from the Menorah? The Baal HaTanya tells us that a little light drives off a lot of darkness. And we know that a mitzwah is called by the name "candle." (כי נר מצוה ותורה אור) With each mitzwah we perform, we have the potential to grow closer to HaShem and draw more of His light into ourselves; to become a little bit more a western candle that shines light outward onto others.