One way to think about this \\(f\\) is to imagine it represents a machine or process that turns Z into X. So given \\(z\\) amount of Z, \\(f\\) answers whether or not we can make \\(x\\) amount of X. But the machine requires electricity (Y), but also happens to produce electricity.

Thus, \\(f\\) depends on whether or not we have enough electricity. But if the machine makes less electricity than it needs (which must be true if the 2nd law of thermodynamics is true), then we cannot make X given Z for any amounts of X and Z. We need an external supply of energy.

Alternatively, if the machine makes more electricity than it needs, we don't need to specify how much electricity is supplied as an input or demanded as an output. The machine will run regardless if there's enough Z to make X. Maybe Y is money instead of electricity if you want thermodynamics to hold, and the machine costs \\(y'\\) to run but tourists pay \\(y\\) to watch it run (a contrived example to be sure).