Indispensable when you shoot in low-light conditions. You can do it in Photoshop with filter and some manipulations. I have found it easier (can’t say better, since I have not done any tests) to use a dedicated sw to do it. (BTW, if you are going to use noise reduction, you should do it at the very beginning of the development process.)
Exists in a stand-alone version and as a plug-in to Photoshop. I like the stand-alone better, and it has a batch function that works well. User instructions aren’t always obvious though. It turned out (if I understood things correctly) that the sw can not choose a built in noise profile based on EXIF from a tiff file (but you can do it with jpg I believe) even though you are given the impression it works. I resorted to always letting the program auto-generate a noise profile. Works pretty well.