Is it really so hard to figure out why it is necessary? Forget moderation for a while. Being an administrator is a lot more than moderating posts and fixing broken ****. People tend to often forget that, sadly.
As you should already be aware, people possessing adequate technical skills are hard to come by, yet are critical in keeping a community operational. Thus we should all do them a favor and not expect these same people to handle everything from technical tasks to mundane tasks.
This is where community admins come into the picture. It is far easier to find dedicated, level headed and reasonable people. than it is finding people with these same qualities plus required technical skills. They handle all these mundane non-technical tasks. Freeing tech admins from the necessity of attending the community in non-technical matters and allowing them to focus their time on improving technical side of things when no things are broken. You know, wouldn't it be great to actually improve the services provided or introduce new services rather than keeping status quo year after year?
Both tech and community admins are a necessity to provide a functional, well provided community. Even one admin dedicated for the task of being a community admin can really help out all the other technically oriented admins. How many community admins are needed is directly proportional to size of the community obviously. HLP would get by with just one or two, always good to have redundancy in times of absence, even if not strictly necessary.
Community admins are not moderators.