The primary reason to stack switches is to simplify management. When you stack switches together they form a "virtual chassis". For example if you had 8 switches, you would normally have to give each an IP address and manage each independently. If you stack them together, all 8 switches act as a single entity with only one IP address to manage. If you only have a few switches it may be unecessary, but in medium and large environments it makes things a lot easier.