Why even a single ship zigzagged against submarines

What benefit did zigzagging give a single warship rather than a convoy?

It disrupted the submarine's approach and prediction of the ship's future position, reducing the chance that straight-running torpedoes would meet the target.

A submarine normally tried to manoeuvre ahead of a target and fire torpedoes toward its predicted future position. Irregular zigs and zags changed the target angle and forced the attacker to revise that prediction while moving more slowly underwater than the surface ship. Because contemporary torpedoes generally ran on a set course rather than homing, even a modest error could turn a hit into a miss. The method therefore helped individual warships as well as convoys by making ambush geometry less reliable. The logic is less applicable to modern homing weapons, and constant manoeuvring can now create its own costs.