Electrolysis of water - why not pure water?
The terrible conductivity of water can explain this.

Pure water is a really bad electric medium, since it has a low tendency to ionize by itself (we know the low concentrations of ions involved by looking at the Kw).

This means that the voltage needed to get a current flow from anode to cathode would be really high, with consequences both in term of safety and energetic waste.

A salt dissolved provides ions, which make the diluted solution a good electric medium, and allow a current to easily flow between anode and cathode.