These are generally referred to as internet 'memes', you can look up meme in wikipedia, it gives many famous examples. The decent ones are all regged in dot.com at least. The long clunky typo-ridden hard-to-remember ones are usually still available to reg.
Most popular memes get their dot.com regged. A meme can become popular for a few days or weeks with some decent search numbers... or it can explode and get tens of thousands of searches in the first month or two, and then slowly wane... but some of them will retain thousands of monthly searches for years afterwards.
Domainers grab meme names to build minisites or blogs, hoping to cash in (with adsense usually) on the search traffic. It's also a hope that a meme will become known in the mainstream world and have lasting usage, in which case a meme term might develop some value to end users who wish to develop something cool/quirky around the term.
Memes are highly risky to invest in, so if you like them then get the occasional one that sings out to you, but I recommend not going hog-wild and regging a lot of them. New ones pop up practically daily, you could spend a ton of time/money regging them, developing some out, and literally have none of them ever come to any value.
IMO their only real value is in any traffic you can get to a site and in how well that converts to ad clicks. There may have been meme sales to end users but I have never personally seen one reported.
Note that I always try add some redeeming value to any of my sites, even minisites. If you grab a meme name and want to develop, try add some interesting and entertaining content yourself; a dry site describing the meme will get one-time visitors, a neat site adding some extra value/entertainment will build strength and garner return visitors and referrals.
I see a lot of memes that I can just tell will be flash-in-the-pans... but when I see one that looks like it might become popular for an extended time, I'll search the dot.com... but so far every one that (I consider) is good, has been regged
