What's the difference between the wire needed for an electric water heater and an electric range, besides amp rating?

The electric range generally needs a neutral wire, and the electric water heater doesn’t. Both run on 240-volts, which doesn’t require a neutral wire. But, electric ranges generally also have a clock, timer, etc. that run on 120-volts, which does require a neutral wire.
A 30-amp electric water heater needs a 10-2 wire (with ground). There’s no place to attach a neutral, so both wires (black and white) are used to supply the power, with each one carrying 120-volts. Electric ranges generally need a neutral wire, so you’d use a 10-3, 8-3, or a 6-3, depending on the amp requirements, with the red and the black wire supplying the power.

Wiring - page 87