Why is it used "it be" instead of "it is" in this sentence?
The purpose is to express something, whether it be feelings, thoughts, or emotions.

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It's present subjunctive, and we use "be" to mean something could be any of these: feelings, thoughts, or emotions. By using "is", we confirm them as realities, when it's simply not the case.

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By the way, "it" is only used in place of a noun in English. You could say "Why is it used instead..." or "Why is "it be" used instead...", but you can't say both.