To Protect and to Serve
Someone once pointed out a connection between what is written on police cars in Los Angeles and the special role of the Kohanim and Leviim. The cars all have written on them, “to protect and to serve.”
This sums up both the service of the Kohanim and Leviim in the Beit Hamikdash, as well as in communities all over Israel.
There was actually a form of guard duty, protecting the numerous entrances to the Temple. If a Levi (usually the ones who couldn’t sing) fell asleep on the job, they would set his clothes on fire. So they literally protected the Temple from trespassers or individuals who were not ritually pure, and could not enter the Temple Mount.
The Kohanim only served in the Beit Hamikdash two weeks out of the year and the holidays. The rest of the time was spent educating the nation and trying to uplift them spiritually. Many of the Kohanim served as judges in the various courts in Israel which included the Great Sanhedrin in Jerusalem.
In short, the Kohanim and Leviim were the spiritual leaders of the nation. They were supported by the rest of the Jews so that they would be free to do their holy work. They indeed fulfilled that role of, “to protect and to serve.”