TT #371 · Parashas Shoftim · Seventh Aliyah · Passuk 21:05
And the kohanim, the sons of Levi, shall approach, for them the Eternal your God has chosen to serve Him and to bless in the name of the Eternal, and according to their word shall be every dispute and every plague.
Torah Temimah points to Sifrei Devarim 208.
TT lemma: כִּי בָם בָּחַר ה׳ אֱלֹהֶיךָ לְשָׁרְתוֹ.
English: ‘For them the Eternal your God has chosen to serve Him’ – this is the service.
Derash on the reason for the kohanim’s role (Sifrei) — their election to serve and bless qualifies them for this rite as well.
Because Hashem chose the kohanim to serve Him and to bless in His name, they are required to be present at the eglah arufah.