Which service is the Pasuk referring to?
Rashi and Seforno: It refers to guarding the Mishkan,
the onus of which lay on the shoulders of Aharon,
and whom the Levi'im were now appointed to assist, to make sure that no Zar enters.
To make sure that no Zar enters it.
As the Torah writes in 18:1. The Seforno adds that they were also appointed to assist Aharon and his sons, on whom lay the onus of guarding the Kehunah, to make sure that no Zar usurps it.
As the Torah continues in the following Pasuk.
Why did the Mishkan require guarding?
Ramban (in his introduction to Bamidbar, DH 'v'Hinei', citing Sifri Zuta): One cannot compare the palace of a king, which has guards, to a palace that does not.
It is a matter of regal prestige.