The number one use of any channel mixer is grayscale conversion. Quality grayscale conversion takes into consideration that the human eye can perceive the same physical brightnesses differently (e.g. blue much darker than green or red). Furthermore, often different colors shall get different weightings when producing the grayscale version in order to enhance contrasts. This is all possible with a channel mixer using the monochrome option. SF GrayMixer already comes with this mode built-in and therefore is easier to use than SF ColorMixer. Moreover, there are additional functions like by-standard luminance conversion, the possibility of fixing a color and a clipping warning.
Some more features:
Lab and CMYK support
support for 16 bit
all settings loadable/savable
For details please see the description here