What is the function of the electron transport chain in cellular respiration. why is oxygen needed for the electron transport chain?

1) The function of the electron transport chain is to pump protons in the mitochondrion inter-membrane, thus building up a proton gradient. This gradient will allow the ATP syntheses.

2) Why we need oxygen for the electron transport chain:
 At the end of the electron transport chain is the Oxygen that will accept electrons and picks up protons to form water. If the oxygen molecule is not there the electron transport chain will stop running, and ATP will no longer be produced. Basically, we need the oxygen to produce more ATP.