The tribe of Levi is excepted from the initial counting in Numbers: it is kept apart from the others. Levi is not one of the tribes designated by flags, nor is it counted with the others. Clearly, Levi stands, to some extent, outside the normal hierarchy.
We know that Levi’s tasks required a certain self-limitation. A normal Jew has a very wide range of ways in which he can choose to serve G-d, to express his personal relationship. A Levi, on the other hand, has a much more limited set of options. He cannot choose to improvise; he must serve G-d, and do so according to the specific and unforgiving rules governing the treatment of the tabernacle and all its appurtenances.
Levi’s role was to provide an interface layer between G-d and the rest of the people. Levi was named by Leah “so that my husband will escort me” (Gen. 29:34), and Levi’s job is to act as an escort for the divine presence. An escort’s task is quite distinct: just as a bodyguard or mourners accompany and guard their charge, an escort walks alongside, shadowing without losing themselves. Levi must stand apart from both G-d and the Jewish people, capable of sufficient distance that they were able, in the time of the golden calf, to slay their fellow Jews.
It is interesting that at the Golden Calf, the tribe of Levi do not take the initiative by destroying the false worshippers before Moshe calls on them to do so. I think the reason for this is closely linked to their role: an escort does not lead – he follows. Just as the massacre of Shechem was described as being done by “Simeon and Levi”, so, too, the tribe of Levi did not act until Moshe called for supporters. Levi are not afraid of action, but they are followers, not leaders. Serving G-d in the tabernacle is no place for individual style or initiative. It is a place for superb followers.