Its blood contains unique medical properties that are in very high demand; their blood is copper-based instead of iron-based, with hemocyanin instead of hemoglobin, which is what gives it its distinctive blue color.
However, the real value does not come from it's regal blue blood, but rather a protein it contains that it contains that can detect Lipopolysaccharides (LPS) at very low doses. To poorly summarize, LPS are component of gram-negative bacteria who differ from their gram-positive counterparts due primarily to an extra, impermeable layer; which makes them more resistant to antibiotics.
When horseshoe crabs encounters this kind of bacteria (such as E. Coli or Salmonella), its blood coagulates instantly into a sort of gel, making it an impressive natural defense, and a conveniently excellent detector for scientific experimentation, especially given that these sort of bacteria are often lethal to a variety of organisms, even at low concentrations.
It is important to note however, that while the blood is therefore pretty valuable, it is only valuable it has been adequately processed and the protein extracted. The blood itself is not worth much without processing, but that hasn't stopped the predictable rise of poaching. The market must respond as it must, but to whoever is reading this, please don't take this post as an invitation to try to profit by harvesting horseshoe crabs; I happen to think they're pretty cool, and I'd prefer their farming to be left to scientists, especially ones who also think they're pretty cool.
Still, I find this situation to be interesting, in that it seems to be illustrating the pecuniary externalities of evolution.
Darwinian competition has long since proven an excellent driver of technological advancement (apparently death and sex are extremely good motivators), and as a result, nature has managed to find brute-force solutions to many problems that that we are simply not prepared to approach, and often with such an eloquence that it often makes us jealous.
From super spider silk to antimicrobial shark skin, we are only just beginning to comprehend the abundance of technology that already surrounds us, and as we learn to copy more and more of nature's shortcuts (the fancy term for which being Biomimicry), we will probably be seeing quite a few interesting human inventions that like a lot like stuff we'd find in nature; like these 7-foot tall crab robots for example.
However, that's not what we're doing here, we aren't trying to mimic Horseshoe crabs, we're trying to borrow what Horsheshoe crabs are made of.
As we get faster and faster genome sequencing, it is only logical that we start to notice the wealth of genetic shortcuts that are already around us; why copy nature's phenotype if we can copy natures genotype? I wonder then, in this coming theoretical arms race that is genetic monetization, could there be a correlation between evolutionary success and medical profitability?
Well, maybe. See, Horseshoe Crabs have been around for nearly 450 million years, and they've barely changed; their lack of evolution implies that they have reached a more or less optimal adapativity for their chosen environment, and barring significant outside forces, will probably remain more or less unchanged for another few million years (knock on wood.) Now, I'm not saying that the older something is, the more valuable it must be, economic value is derived at the nexus of technology and human usefulness, and therefore cannot be predicted so simply (we can't predict what genes might be useful until we know what we want to use them for).
However, what I am saying is that anything that managed to outlive the dinosaurs is probably very, very good at doing what they do (even if it's just chilling out and eating things), and these veterans probably have a few tricks to teach us about life and survival.
In any case, the one thing I can be sure of is that the keys to our continued survival are probably already around us, and the people that reach them first are going to be in a key position to dictate how our species advances; a realization that I'm certain someone at Google has also figured out.
In conclusion, I believe we are on the verge of learning how to read mother nature's cheat sheet, and and I do not yet know what the consequences of that will be, but I happen to believe It is in our nature to cheat nature, and I can't wait to see where we'll go next.