The White House has been a home to a number of smart people worthy of being called the President of the United States, and the ranking by IQ is a way to place them in a sort of a hierarchy of smartness. We love hierarchies after all. If we didn't, we wouldn't even have a president.
The fact that some presidents may have a higher IQ than the others does not imply that he is better than them - IQ measures only certain types of cognitive abilities. Although a low IQ may not be a sure-sign that you'd be a bad president, but I'd rather have a president with a high IQ.
The term "IQ" tends to be associated with political figures such as Abraham Lincoln, Winston Churchill, Barack Obama and others who all had notedly high IQs, while there are other presidents who aren't known as well for their intelligence. Don't be too shocked when you see where Trump is ranked!
