In the beginning the Avodah (service) of Karbonos was with the bechor (first-born -- see the mishnah in the last perek of Zevachim. The avodah was done by the bechorim until the Mishkan was built) ...
At a parshat hashavua shiur that I regularly attend, the rabbi has stated on multiple occasions that the bechorot (firstborn sons of Israel) were supposed to serve the purpose that the Leviim and ...
The Leviim wash the hands of the Kohanim when they go up to bless the people. If a Levi is not available a first born (to the mother) can wash (according to the Askenazi custom) instead, Orach Chaim ...