Color mixing gives you a range of colors with a minimum number of tubes of paint (very useful when painting outside your studio). If you're using a lot of a certain color, you'll probably decide it's easier to buy it in a tube rather than mix it up again and again.
But you'll find that there'll always be an instance when the color you want simply doesn't come ready-made, such as a particular green in a landscape. Your knowledge of color mixing will enable you to adapt a ready-made green to the shade you require.
The advantage of buying a premixed color is that you are assured of getting the identical hue each time. And some single-pigment secondary colors, such as cadmium orange, have an intensity that's hard to match from mixed colors.
