If you have the kind that I have (this kind), you put water in the bottom, espresso grounds in the middle, and put it on the stove. The steam from the water percolates up through the coffee, and boils out the top pipe, into the top compartment. When it stops bubbling through, or when you have enough espresso, you take it off the heat and pour out the espresso from the top. You have to essentially boil it, because how else would the water get from the bottom to the top, except in the form of steam?
The essential brewing characteristics of espresso, as I understand it, are that it’s brewed under high heat and pressure. You can see why steam-brewing it would provide both conditions.