"be of"

It is used to aid something/someone.

This can be of help to you.
Your knowledge can be of importance to your students.
The clerk asked: How can I be of service ?

In your sentence example "to be of value" means it's not clear, if the
ointment is really helpful in preventing infections.

Thank you again.

You are splitting the phrase at the wrong place. It is not (are of) (value) it is (are) (of value).

To be..

of value - to be valuable or helpful or useful
of use - to be useful
of assistance - to be helpful
of good cheer - to be happy or hopeful
of importance - to be important
of significance - to be significant
of sound mind - not insane