Building a PC has many advantages over buying one in a store. You can customize it to anything that you want. It could be focused on gaming or CPU speed, or it could be for everyday tasks. It is also MUCH cheaper that buying one. If is easier to upgrade after you build it and you know how much power it has (Watts). This is useful so you don't buy a cool new graphics card, and realize that your power supply isn't powerful enough to support it. You also have the satisfaction of knowing that what you built was made by you.