Alternative electron acceptors are necessary in prokaryotic metabolic flexibility. Being able to use different terminal electron acceptors allows _E. coli_ to grow in a wide range of conditions (reducing, oxidising). In anaerobic respiration, oxygen is not present and so cannot be used as the terminal electron acceptor.
_E. coli_ is a facultative anaerobe, able to grow in aerobic and anaerobic conditions. This requires a high level of flexibility.