One reason why this is common would be that many (most common?) e-mail clients on Windows don't do proper threading based on
Proper threading also requires that the previous message in the thread is available; depending on the mail storage design, it may even need to be available in the same mailbox. On mailing lists (reflectors) this is a minor problem; for one-on-one conversations between individuals, it's a game-changer.
Having a visual cue in the subject line allows the user to see easily, at a glance, from a single entry in the list of emails, whether a message is an original message, a forward, a follow-up or reply, etc.
Forwarded messages commonly don't include any reference information; the MUA would have to go hunting through any mail message-parts, assuming that the mail is even forwarded as a MIME attachment and not simply copied by the MUA into the body of the new email.
Changing convention is a hard thing to do.
All of the above seem like good reasons to keep the tags in the subject line even if technically they could be omitted in some cases.
That said, I agree with you on the translation of those tags, which can create a mess. Not only can the recipient be unfamiliar with the tag the other person's software uses (I'd have no idea what
İLT means, for example, let alone
תגובה which I doubt I'd even recognize), but some combinations are outright ambiguous (Danish/Norwegian
VS: and Finnish
VS:, for example), greatly reducing the value of the subject line tag. Wikipedia: List of email subject abbreviations in languages other than English.