https://cgcookie.com/tutorial/denoising-renders-in-blender-2-81
Because to me they all do the same thing.
There are two Denoisers and they both reduce the noise, but in a different way. It’s like saying that Cycles and Eevee do the same thing, Yes, they both render out an image, but in a completely different way.
Watch the above-mentioned video (JL explains it way better than I can and I think he covers everything you want to know) and come back if you then still have questions.
