The
Background Sync API gives developers the chance to synchronize data with their server when Chrome (or the device) regains connectivity. The content setting you refer to enables you to disable this behaviour, either globally or per website.
A practical example would be an offline enabled chatting web application, in which you type a message to a contact whilst offline. You usually want such messages to delivered as soon as the device comes back online -- that's why it's enabled by default. If, for any reason, you don't want a website to be able to do this, you can change the setting.
Thanks,
Peter