Spot welding is typically used when welding sheet metal. Thicker stock is difficult to heat up from a single spot, as the heat can flow into the surrounding metal too easily. You can easily identify spot welding on many sheet metal goods, such as metal pails.
Perhaps the most common application of spot welding is in the automobile industry, where it is used almost universally to weld the sheet metal forming a car. Due to changes in the resistance of the metal as it starts to liquify, the process can be monitored in real-time to ensure a perfect weld every time. This allows the spot welders to be completely automated, and many of the industrial robots found on assembly lines are spot welders (the other major use being painting).