What is the function of もし？

Question

I know that もし translates to "if", but I dont know exactly when or how it should be used. I was reading Yoko Hasegawa's Japanese: a linguistic introduction. In the chapter on conditional clauses, some sentences begins with もし, others dont. Example: もし電気がついて（いれば・いたら・いる　なら）、彼は家にいるだろう。versus 会員（で　あれば・だったら・なら・だ　と）、割引があります。(the original text is in romaji, I transcribed to hiragana for better understanding)

Since たら, なら, ば or と already express the idea of conditional, what is もし doing here? Does it have any additional function beyond those in the 4 conditional particles? If it does, what is this function, and if it doesnt, why is it included?

Answer 1

「もうし」の音変化》相手に呼びかけるときに言う語。「もし、そちらの方」出典：デジタル大辞泉（小学館）

As the above, もし itself is a word like "excuse me". However, it ideomatically functions as an indicator that tells that a conditional clause follows next, which helps you comprehend what otherwise wouldn't be revealed until the last conjugation.

The same thing can be said about あまり、別に、何も (adverb) for partial negation, 決して for permanent negation or なぜならば or なぜかと言うと for reasoning clauses, the last one of which is also the same in the point that it doesn't have extra semantic value besides what's combined with.

This type of words are called 呼応の副詞 (Guiding Adverb).

Answer 2

One notable difference is that it disambiguates たら. This is because たら can be used to mean "if", and it can also be used to mean "then". However, if you combine もし and たら, then it becomes clear that it has the meaning of "if".