Basically, its so you can drop a gear and get a smooth gearchange. Say you are going up a hill, the car is slowing down and you need a lower gear to make it. Instead of dropping a gear and immediately flooring it (will result in a jerky gear change) you blip the throttle to rev match the new gear with the same speed you were doing in the previous gear. If you do it quick enough and know what you are doing you can do it perfectly and not even feel the downshift. Its the same with the heel and toe, if you do it right and match the revs properly, you can get a gear change and not even feel it, then get back on the throttle quicker.