i think they serve the multi-eternity as the celestials individually can be in two or more places simultaneously to judge planets across marvel.
after all, there is only one arishem and judging a planet takes the host upwards to a millenium to assess.
and in our real life universe, scientists have discovered tens of thousands of far off planets in the milky way galaxy, now factor that into the undiscovered planets and the thousands of galaxies in the virgo super cluster and 10 million clusters in the known universe.
( than in marvel, maybe thousands of universes)
thats a lot of planets to tend to, celestials have to be multiversal