Generally, these structures are used to place more emphasis on what you want to say. You can say “it is of great importance/ it is of great interest/ of (no) use etc. Sentences like It is very important/ interesting/useful etc will sound less emphatic.

Hi Tamas, I think OF is considered a preposition because it is used to show that a person or an object belongs to, or is related to, something. For example, in the sentence: “The first page of the paper provides the history.” The word “of” indicates that the “first page” belongs to/ is connected with the “paper.” Definition: a. belonging to, relating to, or connected with Examples: It was an act of bravery. That was very nice of you.