Variable pitch blades are usually preferred in most applications because of the varying teeth configuration per inch. Beacuse the teeth vary between fine and coarse, sawing harmonics vary, thus vibration and chatter are dampened. Adjusting the blade speed is another way of reducing sawing harmonics problems. Think of a washing machine that speeds up during the spin cycle. At a certain point the rotation of the drum hits "resonance" and the washer want to start walking across the room. Soon the rotation speed increases and the washers stops walking and shaking. You don't want to have your saw hit resonance and stay there.
Variable pitch blades accomodate a wider range of material cross sections and are particularly useful when cutting rounds and tubing, due to the fact that the top and bottom of the curved workpiece will have a smaller cross section than the middle of a round solid and this is where the harmonics problems tend to be the worst (entering and leaving the workpiece). For tubing and pipe, the top of the material cuts like a solid, then the sides of the tubing cut like thin flat stock.