Aryl chlorides and bromides can be easily prepared
Question:

Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine respectively in the presence of Lewis acid

catalysts. But why does the preparation of aryl iodides requires the presence of an oxidising agent?

Solution:

Iodination of arenes is reversible due to the formation of HI. To move the reaction forward, an oxidizing agent like HNO3 or HIO4 oxidises HI, thus stabilizing the product.