Without it, the computer could not operate. Above is a picture of the 440BX Chipset. Both chips share the task of controling the PCI and memory data flow. In the diagram below you can see the data flow paths mapped out for the chipset. Notice the paths managed by chip 2 are blue.
Buses are simply the paths, or wires, that connect one component to the other.
You may have also noticed that the chips in the chipset are refered to as bridges. The chips actually are bridges that bridge the components together, and ensure that the data flow is directed to the proper place. The better chipsets are those that can handle data the fastest. Below is a picture of both chipsets on a motherboard.