This has a number of advantages over the traditional stretched canvas, for example it makes the painting much tougher to accidental physical damage, and eliminates atmospheric moisture getting into the back of the canvas   

With the passage of time an oil painting becomes more brittle, the oil becomes rancid and acidic, as it gradually eats into the fibers of the canvas, making the painting more vulnerable.  In additional to these problems canvas has an hydroelastic property whereby it will absorb moisture from the air at the exposed back, this causes the canvas to expand, that is why we sometimes see ripples on the corners of oil paintings, and to counter this in each of the four corner joints of the stretcher frames there are inserted two wedges so that the frames may be expanded and the canvas slack taken up.

When the canvas is bonded onto MDF, as I do, all of the problems mentioned in the above paragraph are eliminated, and I have never experienced any negatives.  This means that the owner of one of my paintings can enjoy having it hanging on the wall for many years to come without having any worry about it becoming damaged, or distorted because of tightening of the wedges.

As mentioned in the first paragraph my medium is oil, I firmly believe that is always better to invest in the very best quality of paint available and that Is why I mostly use Michael Harding's oil paint with some Winsor & Newton artist grade oil paint.