Confusion about a step down transformer use
In the USA there are various common AC (60 Hz) voltages distributed to residences and business. These voltages are typically either three-phase (industrial &amp; commercial customers) or single-phase (residential customers and small businesses). The single-phase residential is nominally 230 volts line to line, and 115 volts neutral-ground to each line. This is fairly universal across the USA for residences. 

In the business and industrial sector there are many more types of distributed voltages, most of which are three-phase. Some common voltages are 240 and 440. (these are all 60 cycle, and expressed as RMS voltages) There are other higher voltages distributed to industrial customers, again typically three-phase.

So, to answer your question, when you need to power a 115 volt device, and all that is available is 220 or 440, this is one situation where the transformer you reference would come into play.  

In general, a 50 Hz transformer can be used with 60 Hz power, but a 60 Hz cannot be used with 50 Hz power. So, that's another factor that might come into play if you wanted to use an "American" transformer in Europe.