The List-Unsubscribe header (and its companion, List-Unsubscribe-Post, for one-click support) lets a mail client render a native "unsubscribe" option directly in the inbox UI, backed by either a mailto: address or an HTTPS link the client can POST to automatically.
Major mailbox providers now require bulk senders to support one-click List-Unsubscribe, and treat its absence (or a broken implementation) as a deliverability risk factor independent of complaint rate itself.
A working List-Unsubscribe header measurably reduces spam complaints, because recipients who can unsubscribe in one click from the inbox UI are far less likely to reach for the "report spam" button instead.