"Be Of " style

Here is the sentence

Seller's (a person's name) story is of a beneficial cascade.

why there is of in the sentence, I think it is the same without the "of"

I cannot find "be of" as collocation in the dictionary. But I have seen some articles use this style

For example

Airconditioner is of no use this winter, as it is warm.

This sentence is made up by me, so it might be wrong.

Thanks for any help for analysing this.