My friend and mentor David Ogilvy once took an IQ test later in his life.
He tested 95 and it bothered him.
However, it didn't stop him from founding Ogilvy & Mather, attracting stellar accounts such as Rolls-Royce, Royal Dutch Shell, Unilever and American Express, innovating the industry as a whole and playing a fundamental role in building the agency into one of the foremost ad agencies in the world with billions in annual revenue.
He was also one of the most astute, incisive and intelligent men I have ever met.
As Liang-Hai Sie correctly pointed out, it's mainly an indicator of how good you are at taking tests.