The necessity of interpretation comes back to the necessity of knowing the Qur'an. The Qur'an should be known and explored in its corners in order to get to its depth and content and take advice from its guidelines. The Holy Quran has spoken about the importance of thoughtfulness and thinking in its verses and has encouraged Muslims to study and think about its words. In one place he says: This is a blessed book which We have sent down to you so that you may ponder over its verses and the wise may take advice.
On the other hand, it is true that the Qur'an was revealed in the language of the people and it is a clear book, but the Prophet was ordered to explain the Qur'an, , so that people can learn what was revealed to them. Because understanding the appearance of meanings is different from interpretation. Anyone who knows Arabic can understand the surface of the meanings, but can't reach its depth except with effort and having the necessary tools and knowledge.
Apart from this, if someone carefully pays attention to the contents of the Qur'an, he will realize that the Qur'an has its own characteristics that require the necessity of interpretation in every age and time.