WHY IS 'HEN' ADDED BEFORE ADJECTIVES ?

It's like "quite", "rather", or "very".

Actually, this is just a language habit. Although, 很 means very, some times, when we say我很累，we actually don't mean I'm very tired.

treat the 很 as the word 'quite', may be easier to understand. most of the time, 很 is used as an adverb. means to intensify of something or some state.

It's placed before adjectives, not with "very" meaning, but because many adjs in Mandarin has one syllable, so it makes that adj become two-syllable phrase. Otherwise, if you're to compare between two things, you need not “很”, as in example: 台北的春天不冷不热，很舒服。 The correct translation for "very + adj" is 挺……（的）

很 is put before a subject and an adjective to form a descriptive sentence, indicating how the subject is, like"他很高。天很蓝。"Such sentences Without 很 like "他高。天蓝。"are correct too, but the meaning is defferent than the sentences with 很. Sentences without 很 like "他高。天蓝。" imply an contrast, somebody else is not tall if 他高, and something else is not blue if 天蓝.

Placed before an adjective, it forms an adjectival phrase. It means "very much" or "extremely" and indicates high degree. The "很" is pronounced strongly.