What is the use of the soft sign in front of (soft-indicating) vowels, in words like "компьютеры" and "пьёт"?

Question

I was told that the soft sign makes the preceding consonant soft. I understand why it used at the end of words (царь, гулять, здесь) or before consonants (огоньки, письмо). But why is the soft sign used between a consonant and a soft-indicating vowel (like ю and ё) in words like "компьютеры" and "пьёт"? Wouldn't the consonant before the soft sign already be pronounced soft because of the soft-indicating vowel coming after it?

I read it has something to do with "iotation", but I couldn't figure out what that means.

Answer 1

A soft or hard sign before a vowel indicates the palatal approximant [j]. This is the first sound in the English word "year" which makes it distinct from "ear".

Russian words лёд "ice" and льёт "(he) pours" are pronounced [lʲɵt] and [lʲjɵt], appropriately. They only differ in that one sound.

Hard sign in modern Russian is only used on morpheme boundaries, separating the roots starting with е, ё, ю, я from some preceding morphemes ending in a hard consonant; and, sometimes, in transcriptions.

Usage of a hard versus soft sign is, hence, etymological rather than phonetical. Techically, one is supposed to pronounce a palatalized (soft) consonant before a soft sign and non-palatalized (hard) before a hard sign, however, in practice, many Russian speakers soften consonants preceding [j] even if it's denoted by a hard sign. Most Russian speakers would pronounce the first consonant in адъютант and адью exactly the same.

Answer 2

After soft and hard sign and after other vowels, vowels е, ё, ю, я are iotized and read [й'э], [й'о], [й'у], [й'а] respectively.