This backstory explains why Sanji hid his origins for years and frames the present-day arc: the forced marriage forces the secret back into the open.
The payoff
Why this matters
A father’s disownment-by-faked-death is the hidden engine of Sanji’s secrecy and his estrangement from the Vinsmokes. The WCI resolution lands Ch. 870: Sanji renounces the Vinsmoke name and parts with Judge on his own terms.