Toning a canvas is a process that dates back to the very beginning of oil painting. Other than tradition, I have several very practical reasons why I tone my canvases. First let me say that toning is not absolutely necessary. Many of the Impressionists preferred to work directly on white canvas. I will sometimes start a plein air piece on a white canvas but even then I find I start with a monochromatic underpainting that essentially acts as a toned canvas.
There are several ways to approach a toned canvas. Some artist have a canvas toned and dry before they lay their sketch in. I have done this at times but I find I prefer to have more light show through the canvas in the lighter areas. Almost always I combine my underpainting with the toning process. Much like using vine charcoal, I build my mid-tones and darks and wipe away my lights. Once this is complete I let it dry and it is then ready to paint on. The same process can be accomplished en plein air by using a quick dry medium. Don’t forget your rules about fat over lean!
So now that I have explained my process back to the why? I find the white of the canvas distracting. It makes my lights look dark and my darks almost black. On a toned canvas the intensity of the white canvas is knocked down and I can see my colors more clearly. A toned canvas unifies the painting. The same tone lies underneath the entire work and affects the color above. I often tone in a color that is opposite of the painting to help the colors vibrate. If I am painting cool painting I might use a warm earth tone. If it is a warmer painting maybe a cooler grayish tone. Combining my sketch and my toning allows me to focus on the drawing and the values. I use this process to establish my lights and darks, again unifying the composition. It is easier to see value when using a monochromatic underpainting. Essentially I find it makes painting easier and quicker.