Since the past progressive expresses an activity in progress at a certain point of time in the past, you use it to describe the circumstances of an event - what was going on when something happened.
  The television broke down when I was watching a film.

I found the passport when I was hoovering the flat.
 
Mom found dad's letter when she was looking for her driving license.
It reminded her of when she first met him many years ago:
She was having lunch in a small café in Chelsea when he walked in and asked if he could share her table. It was love at first sight.
  I didn't hear the telephone because I was taking a shower.

EVENTS AND CIRCUMSTANCES


We didn't go to the football match because it was raining. We preferred to stay at home and play cards. That's how we spent the afternoon.
In stories you use the past progressive to describe background information and "set the scene" for the events.
Unfortunately this is a true story.
In January 1978 the firemen were on strike, and the army took over the job of answering emergency calls.

On 14 January 1978 Mrs Brewin was working in the garden. She was watering the plants. Her cat, Henry, was playing around her.

Henry climbed a tree and couldn‘t get down. Mrs Brewing was desperate. She wanted Henry to climb down the tree and called him, but he didn‘t react.

Mrs Brewin went into the house and called the Fire Brigade. She explained the situation to them and they said that they would immediately come to rescue Henry.  Mrs Brewin didn‘t know that she was talking to the army.

While she was waiting for them to come, she took some fish out of the
fridge, went out into the garden and offered it to Henry to get him down. He was still sitting on the tree and didn‘t dare to climb down.

The army finally arrived.
Mrs Brewing was surprised because she
was expecting the Fire Brigade.

The soldiers took their ladder, put it up and a soldier climbed up and rescued Henry.

Mrs Brewin was delighted and invited the soldiers in for some tea. They stayed for half an hour and had a nice chat with Mrs Brewin. Then they said goodbye to her and went out of the house to their lorry.

The soldiers got on the lorry and as they were leaving, they ran over the cat and killed it. They didn‘t see that Henry was playing around the lorry.