The "why do we twist" question is best answered in the full context of wire selection.
There are three key properties to consider in the design of electrical circuits:
Resistance, capacitance, and inductance.
Resistance. How do we ensure that a wire will not overheat, when we apply voltage and current to it? By selecting a wire gauge that is sufficiently large. Don't use 22 AWG telephone wire to power a room air conditioner.
There are two kinds of electromagnetic induction: Capacitive coupling (electrostatic), and magnetic coupling (electrodynamic).
Capacitance. How do we ensure that the signal transmitted over a wire, will not be degraded by excessive noise, when we place the wire into an environment, that has ambient electrical fields which tend to capacitively couple with the active signal-carrying wires? By covering the wires with a continuous metallic shield or foil, which is grounded at one or both ends. The ambient electrical fields capacitively couple with the shield instead of the active signal-carrying wires. This helps prevent your wires from playing the role of a capacitor anode or cathode.
Inductance. How do we ensure that the signal transmitted over a wire, will not be degraded by excessive noise, when we place the wire into an environment, that has ambient electromagnetic fields which tend to inductively couple with the active signal-carrying wires? By twisting the wires continuously, from end to end. The ambient electromagnetic fields magnetically couple with thousands of very small twisted coils, instead of the entire untwisted "single-loop" signal-carrying wires (a transformer with a 1:10000000 ratio induces infinitely less noise than one with a 1:1 ratio; it's called lowering your effective loop area, ELA). Also, each adjacent pair of twists consists of one oppositely-wrapped loop, so each pair is self-cancelling. This helps prevent the wires from playing the role of a transformer primary or secondary coil.
There are three key properties to consider in the design of electrical circuits:
Resistance, capacitance, and inductance.
Resistance. How do we ensure that a wire will not overheat, when we apply voltage and current to it? By selecting a wire gauge that is sufficiently large. Don't use 22 AWG telephone wire to power a room air conditioner.
There are two kinds of electromagnetic induction: Capacitive coupling (electrostatic), and magnetic coupling (electrodynamic).
Capacitance. How do we ensure that the signal transmitted over a wire, will not be degraded by excessive noise, when we place the wire into an environment, that has ambient electrical fields which tend to capacitively couple with the active signal-carrying wires? By covering the wires with a continuous metallic shield or foil, which is grounded at one or both ends. The ambient electrical fields capacitively couple with the shield instead of the active signal-carrying wires. This helps prevent your wires from playing the role of a capacitor anode or cathode.
Inductance. How do we ensure that the signal transmitted over a wire, will not be degraded by excessive noise, when we place the wire into an environment, that has ambient electromagnetic fields which tend to inductively couple with the active signal-carrying wires? By twisting the wires continuously, from end to end. The ambient electromagnetic fields magnetically couple with thousands of very small twisted coils, instead of the entire untwisted "single-loop" signal-carrying wires (a transformer with a 1:10000000 ratio induces infinitely less noise than one with a 1:1 ratio; it's called lowering your effective loop area, ELA). Also, each adjacent pair of twists consists of one oppositely-wrapped loop, so each pair is self-cancelling. This helps prevent the wires from playing the role of a transformer primary or secondary coil.
