Consonant-Vowel Sequences in Spoken Korean
A lot of Korean grammatical rules are related to the fact that a sequence of sound is easier to pronounce when it is in a consonant-vowel sequence, i.e., when you don’t have to pronounce 2 consonants or 2 vowels in a row.
For example, a lot of Korean particles come with 2 choices, one with a beginning consonant and one without. When a word ends with a vowel, the particle with a beginning consonant is usually chosen to attach to this word. When a word ends with a consonant, the particle without beginning consonant is usually chosen instead. It keeps the consonant-vowel alternate sequence as long as possible.
Another example is that when modifying verbs or adjectives, 으 could be inserted between the stem of the word and the attaching suffix to make sure the consonant-vowel sequence is preserved.
You will come across with a lot of grammar points having such a characteristic, but there are exceptions, so you will need to identify the appropriate choice on a case-by-case basis.