It also, at least theoretically, jet pumps out when main tanks <50% full. Given that a routine day puts you on the ground at say 2T per wing, that typically leaves 200kg in the middle. Our SOPs have changed to carry Flight Plan +250 kg if C Tank fuel required to reflect this. As Jonty says, if you need it, it can be used by switching pumps on rather than relying on jet pumps.