Could you please inform me about the grammatical explanation (and actual meaning) for the usage "of" when it is used in the sentences such as "It is of importance.".

Ellipsis again, I'm afraid. It's short for "It is [a matter] of importance". In this context, "of" has its normal meaning. (Or at least, one of its normal meanings.)

How about

eg. I am of service to youEmotion: smile