比。。。还要
弟弟比爸爸聪明。 Younger brother is smarter than father.
哥哥比弟弟更聪明。Older brother is even smarter than younger brother.
哥哥比弟弟还聪明。Older brother is even smarter than younger brother (?).

哥哥比弟弟还要聪明。Older brother is even smarter than younger brother (?)
What is the function of 要 in the last sentence?

Question

比 。。。 还要
弟弟比爸爸聪明。 Younger brother is smarter than father.
哥哥比弟弟更聪明。Older brother is even smarter than younger brother.
哥哥比弟弟还聪明。Older brother is even smarter than younger brother (?).

哥哥比弟弟还要聪明。Older brother is even smarter than younger brother (?)
What is the function of 要 in the last sentence?

Answer

A 比 B （更）聪明 =  A is smarter than B
A 比 B 还聪明 = A is smarter than B, given that B is already smart
A 比 B 还要聪明 = A is even smarter than B, given that B is already smart ( with some emphasis)

But in real life they are very similar. With or without 要，the meaning of the sentence will not change. I would say 要 emphasizes the fact that A is smarter.

Answer

The main meaning of this useless 要 here -- is to give you the felling "omg, Chinese is so confusing and difficult" while in fact it's as easy as English.