They are more accurate, more easily used and are less fatiguing. A good matcutter will last a lifetime and pay for itself over and over again. Suffice it to say that if you spend the money for good matting materials you don't like throwing it away because you screwed it up using a cheap cutter. Three things will help get better results no matter how you cut your mats. This may mean you change the blade with each cut on a 20x24 board.
Blades are cheap when compared to high quality mat board.
The second item is a solid straight edge. Not one that bows and flexes easily. You are measuring straight lines for cutting. A cheap and lightweight edge will flex and ruin your cuts, whether done by hand or with a cutter. The less expensive cutters flex too easily.