Не могу понять почему это здесь.

Tassador, do you speak English? I ask because based on Google Translate your sentence is a correct translation of the romaji text, but in general the sentence would be read as "sorega nani de aru ka wakaranai" I believe, which has a different (though stilted) meaning. "nani de" would make the sentence mean "what is this" but "nande" means "why". I don't want to merge another Russian entry, as I cannot speak Russian. Someone with a better understanding of Russian and/or Japanese needs to intervene here.

I think the sentence is written ambiguously.

sore ga nande (=dou shite) aru ka wakaranai
I don't know why it exist.

OR

sore ga nan dearu (=da) ka wakaranai
I don't know what is it.

OK, first off the Index line of the Japanese sentence is

其れ[01]{それ} が 何[01] である か 分かる{わからない}

It might not be correct, but it is unambiguously 何 not 何で if the index line is correct.

It also seems likely that it is possible for that to be read なに in context.

うちの文庫本のほとんどにはカバーをしているので、開かないとそれがなにであるかわからない。
Most of my paperbacks are in covers so you don't know what they are until you open them. ← ○
Most of my paperbacks are in covers so you don't know why they are until you open them. ← ×

(Note that it is usual to write なんで in kana, although kanji may be used).

So I suggest changing the Japanese line to
それがなにであるかわからない。
to make it unambiguous.

@nemo
Note that the romaji line is auto-generated and absolutely can't be relied upon to be accurate. You can read my rants about auto-generated romaji on the Wall. ;-)

I know it's unreliable, that was my point. I didn't want to merge the Russian translation if it was based on the romaji, which seemed to be (and apparently is) the case.

Russian «Я не знаю что это.» = «I don't know what it is.».