It's like anything if you are of the right social class being members of the right clubs can open certain doors for you. Political parties like to hear that you are in Mensa.

The impression I get is that it was more relevant before the internet. Like, you'd be living in bumfuck Nebraska and take an IQ test in high school to see if you should be a trucker or farmer, and then a couple weeks later you get a letter asking if you wanna be part of a club of smart people who have a magazine and meet every once in a while.