We use the past continuous to talk about past events which went on for a period of time.
We use it when we want to emphasize the continuing process of an activity or the period of that activity. (If we just want to talk about the past event as a simple fact, we use the past simple.)
While I was driving home, Peter was trying desperately to contact me.
Were you expecting any visitors?
Sorry, were you sleeping?
I was just making some coffee.
I was thinking about him last night.
In the 1990s few people were using mobile phones.
We often use it to describe a "background action" when something else happened.
I was walking in the street when I suddenly fell over.
She was talking to me on the phone and it suddenly went dead.
They were still waiting for the plane when I spoke to them.
The company was declining rapidly before he took charge.
We were just talking about it before you arrived.
I was making a presentation in front of 500 people when the microphone stopped working.