במדבר פרק יח, ב וְגַם אֶת-אַחֶ יךָ מַטֵּה לֵּוִי שֵּבֶט אָבִיךָ הַקְרֵּב אִתָךְ וְיִלָווּ עָלֶיךָ וִישָרֲתוּךָ וְאַתָה וּבָנֶיךָ אִתָךְ לִפְנֵּי אֹהֶל הָעֵּדֻת
The avodah of the Leviim is the “guarding” which they do for the Mishkan in addition to the Cohanim. Plus, their avodah includes unique services, such as the singing and carrying the items of the Mishkan on their shoulders.
The Leviim accept their secondary role which is to recognize that the Cohanim “guard” above and the Leviim “guard” below. Thus, within the Bais HaMikdash, certain chambers exist which have an upper and lower levels as explained in Mishneh Middos.
The Rambam in the laws of Bais HaBechirah, says the mitzvah of “guarding” is done by the Cohanim from within and the Leviim from the outside. This is difficult since this seems to be different than the above Gemara states (higher and lower). This needs investigation.
The Bais Yosef (O.C. 128) writes that he does not know the source for the custom that in Galus at the time of Bircas Hacohanim, the Leviim wash the hands of the Cohanim. (Note: in the Bais HaMikdash the Cohanim would wash their own hands from the Kiyor). Only the Zohar brings an allusion to this custom.
Rabbi Epstein theorizes that from this verse we might have a source. In one Gemara (Kidushin 22a), it explains that washing is a form of service. And in another Gemara (Zevachim 18b) explains that the mitzvah of the Priestly Blessing is also a form of Divine service. And the Leviim’s place is to assist the Cohanim in their service, which in this case is to prepare the Cohanim to do the service of the Blessing by washing their hands.
