This is only number eight on Candy Addict's Top 10 Grossest Candies, and man, they weren't kidding. It's totally worth reading the whole list (link below). I highly recommend it.
As the article points out, these candies aren't gross because they necessarily taste bad or have icky ingredients; mostly, they are just conceptually gross. And that's what makes them so interesting.
I really like the idea that, simply because kids are very weird by nature, there is actually a whole marketing category based on appealing to kids through "grossness." And consider the implications! This means that there must be product developers coming up with this stuff, possibly spending their whole day sitting around boardrooms doing nothing but thinking about gross candy, and brainstorming for hours about every possible way to make candy look as disgusting as possible. Isn't that great? Dream job!
And if there are people like that, what are they like? Do they actually eat the candy? Are they fat? Do they go home to their wives and say, "Honey, I was great today! Henderson loved my idea for the plastic nose that drips candy snot. I have arrived!"
They probably create whole PowerPoint Presentations based upon all the various "gross" categories, like Snot, Vomit, Blood, Bathroom, Wounds, Bugs, etc. Awesome!