Answer:
The oxygen bottles provide an added level of security for Hillary and Tenzing that they will have oxygen available for their descent.
Step-by-step explanation:
Sir Edmund Hillary adds the detail of the partially used bottles of oxygen in the narrative because he wants to share with the audience and give them the feeling of assurance that he felt while climbing the mountain, because he knew that if something went wrong, there would be like a "safe net" that would help him if he ran out of oxygen going down.