Why do we need adjectives with the suffix "-most" and what is there in common between them?

Question

Almost all adjectives below are from wordtoolbox.com.

the adjectives I could pair with each other:

easternmost (*eastmost) – northernmost (*northmost) – southernmost (*southmost) – westernmost (*westmost)
innermost (inmost) – outermost (*outmost)
leftmost – rightmost – *middlemost (*midmost)
farthermost (*farmost) / furthermost – *nearmost
*downmost – uppermost (*upmost)
*lowermost – *highmost
*bottommost – topmost
backmost – *headmost
rearmost – foremost

the adjectives I couldn't pair with each other:

*deepmost
*nethermost
*undermost
*endmost
*hindermost (*hindmost)
*hithermost
*lattermost
uttermost (utmost)
aftermost (*aftmost) / *sternmost

I marked with an asterisk the words there are not in oxfordlearnersdictionaries.com.

---

Is this like the third variant of superlative form along with "-est" and "most"?
Is there any name of these adjectives in linguistics?
Do they have something in common between each other?
Why do we need them if there are usual superlative forms: "-est" and "most"?

As far as I understood from the internet:
"southernest" is incorrect,
"most southern" is incorrect,
"southernmost" is correct.
Based on this, why was the word "southernmost" created if we could use "southernest" or "most southern"?

As far as I understood from the internet:
"highest" is correct,
"most high" is usually incorrect,
"highmost" is correct.
Based on this, if we have the word "highest", why should we invent "highmost"?

Answer 1

There are many bits and pieces of language that we don't really need. English wasn't designed; it evolved.

This particular oddity evolved from an Old English (pre 1200) suffix -mest, which itself is from an superlative form in Anglo-saxon "-uma" combined with the regular superlative "-est". It is unrelated to the word "most".

It now is a vestige; a feature that was once part of normal grammar, but now only persists in a few words, mostly words indicating location.

From your list, many of the words are obsolete in English. We don't say "highmost", or "deepmost", and "downmost" is fairly limited. The directions "Southernmost" and a few random words like "bottommost" have some use, and the exceptional "utmost".

So from your specific examples "Most southern" and "southernmost" are both correct but used in slightly different situations.

The southernmost point of England is "The Lizard". (about a location)

The kiwi is the most southern bird in the ostrich family (about a thing)

"Highest", and "most high" are correct but "highmost" is incorrect. "Most high" is rare and reserved for some formal contexts.