Because Korean letters must start with consonant. So, we need to put ㅇ to comply with the rule. 시 = ㅅ + l (consonant ㅅ + vowel l) 간 = ㄱ + ㅏ + ㄴ (consonant ㄱ + vowel ㅏ + consonant ㄴ, in this case, ㄴ is called 밭침 (ba chim) 이 = zero consonant ㅇ + ㅣ (vowel l) 얼 = zero consonant ㅇ + ㅓ + ㄹ But in 녕 ㅇ is differently used as 밭침 and it has an actual sound 'ㅇ' (ng in English) consonant ㄴ + vowel ㅕ + consonant ㅇ (밭침) So, this is the rule.

When a Korean word starts with a vowel sound, the 'o' is put in the place of a consonant. It has no sound when it's on top of the syllable and only has the 'ng' sound when on the bottom. Vowels can not come first in the syllable and so 'o' is just a place holder.