“Neat Image is a digital filter application designed to reduce visible noise in digital photographic images. It is a tool for owners of digital cameras, flatbed and slide scanners; and is for use by both professional photographers, and digital image processing enthusiasts.”
Of course, you can do all that with Photoshop but Neat Image just looks somewhat easier to use. I’ll have to do more tests to see how good it really is but the tests I did looked all very promising.