The most inhospitable places on Earth usually host some sort of life, from the super-hot hydrothermal vents on the ocean floor to the ice of Antarctica. While not as extreme, caves are also quite inhospitable. Complete darkness serves as a harsh restriction on what can – and can't – survive there.
UV light is dangerous to humans. That's part of the reason why there's widespread interest in discovering light sources that can kill unwanted organisms – while leaving humans unscathed.