Plain version: cancellation closes the door from today forward; rescission treats the door as if it had never been opened.
Rescission undoes the contract. California Civil Code §1688 says a contract is extinguished by its rescission, and under §1691 the rescinding party gives notice and restores what it received, which for an insurer generally means returning the premium. A court order is not required. Cancellation simply ends coverage from its effective date going forward.
An insurer generally has the right to rescind for:
Concealment, intentional or unintentional (§331)
An intentional and fraudulent omission of facts tending to prove a warranty false (§338)
A representation false in a material point (§359)
Violation of a material warranty or other material provision (§447)
The key word is material. Under §448, unless the policy declares that violating specified provisions will avoid it, breach of an immaterial provision does not avoid the policy.