The usage for "be of"
I' very confused about the usage that "of" following verb "be".

If is it some sort of a phrase or idiom?

e.g.

Can I be of help?
- why not say (or if has it the similar meanings) "Can I help?"

More examples, such as

My advice may be of use for you.
- "My advice may be useful for you." ？

I was glad to be of some assistance.
- “I was glad to do some assistance.” ?

The structure is of special importance.
-"The structure has special importance." ?

Question

The usage for "be of"
I' very confused about the usage that "of" following verb "be".

If is it some sort of a phrase or idiom?
 
e.g.

Can I be of help?  
- why not say (or if has it the similar meanings) "Can I help?"

More examples, such as

My advice may be of use for you.  
- "My advice may be useful for you." ？

I was glad to be of some assistance. 
- “I was glad to do some assistance.” ?

The structure is of special importance.
-"The structure has special importance." ?

Answer

To be plus 'of' is just an alternative way to say different things. We can use 'to be' plus adjective. I want to be helpful or I want to be of help or I want to help. They all mean the same. 'To be of' is more formal and a little antiquated since most people don't use it except in a formal context.

Answer

be of help
be of use
be of assistance
be of importance

The examples you give explain all you need to know about this structure! Just follow them!

The advantage of the "be of" form is you can express varying degrees. Compare:
I helped him.
I was of some help to him.
I was of a little help.
I was of a great deal of help.
I wasn't of much help at all.
I was of no help whatsoever.

Ex: It would be of a great deal of help to you if you studied properly.

Answer

When you get confused by a new term, you look it up.

Ne manquez pas cette occasion d'apprendre une langue depuis chez vous. Découvrez notre sélection de professeurs de langues expérimentés et inscrivez-vous dès maintenant à votre premier cours !