Image-to-image starts not from pure noise but from an input image — more precisely, from that input with some noise added, and the strength value decides how much noise that is, which in turn decides how many denoising steps actually run.
Near 0, almost no noise is added, so almost no denoising happens, and the result looks nearly identical to the input. Near 1, noise nearly blankets it, the input's trace fades, and the result behaves close to plain text-to-image.
A low strength suits reworking a sketch or photo into a different style while keeping its composition; a high strength suits using the input only as loose inspiration for something largely new. There is no correct value — it is chosen for how much fidelity to the original you want.
The demo below is a procedural simulation, not real noise injection and denoising — it blends color and shape between a fixed original composition and a completely different one by the strength amount.
When to use
Keep it low to preserve the original composition, raise it for something largely new that only starts from the original. Mid-range values can land in an ambiguous blend of both, so it helps to adjust while watching results.