GENDER purposes

Yes. There is a gender difference in the second person in Hebrew, Arabic, and many other languages. There is no such difference in English, but it hints to the user that GENDER may (and perhaps should) be used in the translation. If it's not needed at all in the language into which you are translating, just write it like in English, e.g. {{GENDER:$1|вас}}.

But I don't think that GENDER is added to "you" / "your" for no reason in English messages. It may hint that English "you" / "your" could have gender difference in the future :-) Also, there are lots of messages having PLURAL in combinations of an adjective and a noun. Only the noun is placed into {{PLURAL:}} while the adjective is not, despite the fact that in some languages (not English), adjectives have different forms in singular/plural.

No. "You" is not likely to have a grammatical gender in English any time soon, and that's really not the reason for adding it to English messages. As I said, the reason is that it hints to the user that GENDER may (and perhaps should) be used in the translation. I did it many times myself when writing code, and other developers do it for the same reason.

It's similar with PLURAL: Sometimes it's not really needed in English, but it may be needed in other languages, so there's only one parameter in English, but there may be more parameters in other languages. It's also helpful for validation: if the source message uses PLURAL, but the translation does not, there may be a problem somewhere, so a warning is shown. Quite often, more words need to be inside PLURAL in the translation than in English, for the reason that you indicated, or for other reasons.