If you get things right, ViewNX-i will produce tiffs or jpg that are exactly like the jpgs straight out of the camera with all the settings you use. Capture NX-D makes it easier to tweak contrast curves and tone curves after the fact if you have missed something at the capture instant. I do all my basic raw conversions in Capture NX-D as it is easier for me to use one application instead of two and Capture can do everything View does, plus some more.
These applications are not image manipulation software, but rather do what a good copyist did in the darkroom in the analog days.
The two parts of the NIK suite that do the most, creatively speaking are Silver Efex and Color Efex. Silver Efex makes it possible to emulate the whole analog monochrome process from a digital colour file. I have written a blog post on how I use this application. Color Efex makes it possible to emulate different film looks and filter effects in the same way.
One big reason for using post-processing rather than generating the monochromes in-camera is that you can moderate the strength of the effects in post.