Message digest interval options in account settings are not visible.

Problem/Motivation

The digest interval options added in the message settings do not appear in the related field of account settings.. There's only one option that is "Send immediately".

What I am trying to understand is; If Admin selects weekly, does the normal user's "Send immediately" selection apply? This field is required by default and every user has to select it to save account changes.
If everyone has to select the "Send immediately" option, what is the purpose of the interval options or the admin's default selection for global?
Why can't the user select the other interval options?

Maybe I misunderstood the purpose here. I would appreciate it if you could clarify.
Any idea,please... Thanks