As far as I understand, "ч" is always soft. The letter "ь" usually denotes that the preceding letter is soft.
These two facts together seem to imply that the combination "чь" (just like "щь") would/should always be redundant, and thus never used or replaced by "ч".
However, one sees this letter combination in the pronouns "чьё/чья/чьи" -- I have not seen it anywhere else in the Russian language although I do not presume to have a wide vocabulary.
Is the reason why this letter combination exists in these three words historical? Or does it actually affect the pronunciation of the words?
(Say compared to "чё/ча/чи" -- of course because of the spelling rule "чя" is illegal but such a combination would be redundant anyway since the vowel following "ч" is always soft.)
Sort of related: "чё?" or "чо?" - which is the correct spelling?