Primary, Log and Curves

In my own understanding, Primaries Wheels are used for the over all image but it blends with the colours of the image itself.

Log is used if you want to edit it by parts, for example if you liked to edit the highlight, you can just use the highlight to adjust it. 

    

     And the curves is also used for the entire image. If they are connected to each other you can adjust the amount of blacks and whites or the luma of the image and also the colours at the same time. You can adjust the RGB individually if you want to use only one of them by clicking any of it. But unlike the primaries wheels, you mixed colours in here only with RGB.