Its to do with elec bus isolation when trying to conserve ships main batteries or depowering busses one by one to eliminate smoke source.Following dual gen failure with no APU,youd be down to batt bus,sw hot batt,stby busses,INV and hot batt bus..to prolong ships batteries(nowhere to land) you may elect to depower further in an effort to save them if in day VMC on top..its complex and rarely taught.737 must have APU for ETOPS so...