You will most likely see a 'step-up' transformer on neon signs. The next time you're at a restaurant or bar, look at their neon sign. You'll see a black box (4" x 4" x 6"w) connected to the sign (usually). This is the transformer, and it takes 120 volts AC and steps it up to 7500 to 10,000 volts AC. This excites the neon gas and causes it to glow.
A step-up transformer transforms voltage to a higher level. Like 120 volts to 277 volts.
It is type of transformer which is used to step up the voltage from low to high value in order to make it applicable for high rated appliances.
Not only for high rated appliances but also for transmission in power distribution systems. It is critical in this role. Power distribution systems depend heavily on step-up transformers to generate the high voltages needed for more efficient long-distance power distribution. (And yes, there are step-down transformers at the other end to make the voltages more "user friendly" and less dangerous.)