Sentences of "of + noun"

Hi,

I see sentences like, "It's of importance" instead of "It's of important," or "I can be of any help to you" instead of "I can be helpful to you." "of" is a very tricky preposition and now I would like to master these expressions. I don't know when I can use them and how.

Is there any name for this structure? so I can google it and study? Or will you tell me any website that I can learn it?

Thank you,

m