I suspect it was far more valuable in pre-Internet non-population-dense America, where it was one of the few effective ways to connect with other smart people. It is extremely obsolete now. But "they" (whoever "they" is in practice) publish some nifty puzzle books.

At the time I'm writing this, yours may be the only answer in this thread that doesn't try to signal moral superiority to Mensa members by piously insulting them. (There are a couple of other comments that I would consider borderline -- not overtly insulting, but carefully suffused with cynicism.)

It means there's a chance you're no longer the most boring person you know.

I thought the reverse.

They get a magazine and the chance to go to meetings to talk to other "smart" people. I know someone who was but quickly let it lapse.

I think it is mostly about bragging rights.

Probably networking (providing social proof to people who are "like you" that they are sufficiently "like you" that they should help you in some way).

It is a nice way to meet other people that are (somewhat) smart (in a new city, or beyond ones social circle). What is especially good about it is the relative diversity of backgrounds that members are from (and different ages).

However the atmosphere at Mensa varies much from area to area (sometimes a bit pretentious, but in other areas very low key, pleasant and surprisingly open). And one should not take it too seriously.

You get the card and can officially list it on your resume.