The Qur’an according to verse 3:7 consists of decisive and ambiguous verses. Both types of verses have an interpretation. The author of al-Mizan defines “interpretation” (at-taʾwīl) as follows:

Interpretation is that reality to which a verse refers; it is found in all verses, the decisive and the ambiguous alike; it is not a sort of a meaning of the word; it is a real fact that is too sublime for words; God has dressed them with words so as to bring them a bit nearer to our minds; in this respect they are like proverbs that are used to create a picture in the mind and thus help the hearer to clearly grasp the intended idea.

– Allamah Tabataba’i

The presence of ambiguous verses in the Qur’an is one of the objections leveled against the Book. Those who object say: “Had this Book been made clear and kept free from this maze of ambiguous verses, it would have served its purpose in a better way, and there would not have been any chance of controversy and perversity.”

Following, is the gist of the explanation as to why the presence of ambiguous verses was necessary in the Qur’an, appearing in Eng. Vol. 5 of tafsir al-Mizan:

The verbal expressions of the Qur’an are like similitude to the sublime Divine realities. Those realities have been brought down, in these verses, to the level of common minds. An average mind does not perceive except the natural phenomena; it cannot comprehend the abstract Divine realities unless they are put in the mold of concrete expressions.

When pure spiritual facts are expressed in terms of body and matter, either of the two things may happen—both of them dangerous:

(1) The mind may stop at those material expressions, taking them to mean natural phenomena. It will thus fail to see the reality beyond those expressions. It will, in short, take a proverb in its literal sense, not knowing that it signifies something else; and that that something is often not shown by its letters. Thus the intended meaning will be neglected. The minds will not try to look behind the screen of the words, as it will not know that it has missed anything.

(2) If the mind realizes that the verse is a sort of a similitude and tries to see beyond the curtain of the words, by removing from it unwanted elements that have no bearing on its intended significance, then there is a danger that it may discard some important element or leave intact some unnecessary one.

There is only one way to avoid these two dangers, and that is to express that one significance in various proverbs, molding each in a different mold—one proverb would contain some details that would be missing from the other, and the former would not have some details of the latter, and so on. In this way, those sentences would, through comparison and action and reaction, clarify each other and all together would show their true significance. First, the hearer, on hearing various expressions, would realize that they were not used in their literal sense; they were like the similitude describing an abstract idea in the molds of various concrete expressions. Thereafter, he would be in a position to know which details were to be discarded and which to be retained—because the essential factors would be present in every sentence, while unnecessary ones would be missing from one or the other.

This device to explain difficult ideas and complicated thoughts is not peculiar to the Qur’an. It is found in every language, every nation and every place. Man, by his nature, knows that if only one story, proverb or similitude is given to illustrate an abstract idea, unessential details would confuse the minds, and might convey to them a wrong meaning. Therefore, he tries to make the audience understand his idea with the help of a lot of stories and various similitude. So that they may distinguish the true significance from the unnecessary details.

It is now crystal clear that it was necessary—nay, essential—that the Qur’an should contain ambiguous verses; and that, the ambiguity should be removed with the help of other unambiguous verses (that is, the decisive verses). 

Reference: Al-Mizan English Vol. 5