Regular wires and cables always have some resistance. So when electric current flows through regular wires and cables, it always loses some energy, It's only a little bit, so we don't usually worry about it.
But if you're the power company, your business is selling electrical energy. If you generate electrical energy at the power plant, and some of it is lost on its way to your customers, then you can't sell the energy that's lost. If you're the power company, then you're shipping ginormous amounts of energy over ginormous distances, and you DO worry about losing any of it.
Strange as it may seem, it turns out that if you ship the electrical energy through the cables at HIGHER voltage, LESS of it gets lost. So the power company wants to send the energy through the cables at the highest-possible voltage. Real power companies use as much as 765 THOUSAND volts to send energy between cities, and 4,800 or 7,200 volts to move it between neighborhoods. (And there's a new long-distance power line in China using 1.1 MILLION volts to move energy a couple thousand miles.)
But this could be a big problem ! You only use 120 volts for most of the things in your house, (or 240 volts for big things like the air conditioner, the stove, or the clothes dryer).
So the power company needs a way to "step down" the voltage from the "transmission line" before they connect the wires into your house.
They use a "step down transformer" to make the electricity suable in houses.
The step-down transformers are the giant square boxes you see on the ground inside the neighborhood power-distribution substation.
And they're the round black cans you see up on the electric poles around the neighborhood.