The modifiers are for typing in japanese.
No idea what the HHKB key does, but otherwise:
The (\) key is for not converting the typed kana into kanji (Basically Letters into Words)
The ( ) key is for converting the typed kana into kanji
The Kana key is for selection which type of kana you want to use (Hiragana or Katakana, they are basically equivalent but you'd use them in different situations and they give off a different tone, closest example would be all lowercase vs all caps)
None of these keys are actually required to type in Japanese, but they all help a lot (Akin to not having the backspace key and shift key, you can still get by perfectly fine with caps lock and delete but its nice to have). Same reason for why the arrow keys are there.