Spiders: We'd Probably All Be Dead Without Them

Why? Because spiders are the dominant terrestrial predators on the planet.
Why? Because there are so many of them. (An acre of English meadow in late summer has been estimated to contain more than 2 million spiders, and it's safe to assume that wetlands and undisturbed forest contain significantly more.)
Why else? They eat a lot.
So what? Because they structure insect communities wherever they occur, spiders play a vital role in the terrestrial food chain. Without all those hungry, carnivorous spiders, insect populations would explode, food crops would be decimated, ecological balances would be ravaged, and humans would probably starve within a matter of months.
Oh.
That's not all. Spiders also spin silk, which biotechnologists are trying hard to synthesize. (You can't "farm" spiders in close quarters; they eat each other.) Each species produces a unique venom. Spiders live almost everywhere, and display an astonishing range of hunting techniques, mimicry, architectural talent, and general adaptive genius. And in case you were wondering where to find more spider species than anywhere else, including the highly toxic Sydney funnel web spider, the place is Australia.