Unfortunately, with the scant data available at present, it is not possible to determine just why and when these Horus kings discovered a Sethian or Horian-Sethian aspect in themselves, complementary to their Horian nature, and were led to express this by a special name.
Source: wiktionary
In his Horian form, the blessed one offers a long salutation to Osiris that uses the expression: “O Osiris, I am thy son Horus. I have come to…” then enumerates all the benefits that he reserves for his august father.
Source: wiktionary
It was also a powerful marker of legitimacy and eternity and joined the Horian rituals, where Horus ascended to the throne of his father, assuming his inheritance after undergoing a trial.
Source: wiktionary