Explain why a comma is needed between the two adjectives unique and complicated

My teacher always told me that it was so when we were speaking we could pause and take a breath. But I think it is a way to separate or sort our sentences without entering a completely different topic

This depends solely on context. If I were to say..

The art was unique and complicated. (You don't need a comma.)

BUT you do in the case below. 

The unique, complicated art mesmerized the crowd. (Need comma)

You need commas between adjectives if when (BOTH) they can be interchanged and there is no words like AND and OR. 

For example, I could also say, "The complicated, unique art..." because it does not change the meaning of the sentence.