A stolen iPhone which is still attached to the original owner's iCloud account is worthless for personal use or reselling purposes (unless you strip it for parts), because at any point the original owner can remotely lock the phone and find its location with Find My iPhone.
An iPhone can only be associated to one iCloud account, meaning that, in order to sell it to someone else (or in order for a stolen phone to be used by someone new) that account needs to be removed from the phone altogether. In 2013, Apple introduced a security feature designed to make iPhones less valuable targets to would-be thieves.