Past Continuous or Past Progressive is usually used with Past simple to talk about actions in the background. It expresses a descriptive past. We usually use it with past simple tense. Past simple tense interrupts past continuous tense, which is an action in progress in the past.
Why do we use it?
We use past continuous to talk about the actions in progress in the past.
Peter was sleeping when the phone rang.
While I was having a bath, Mary called home.
Past continuous is mainly used for describing the background actions in the past. On the other hand past simple is used to talk about the completed single actions.
"It was a great day. The birds were singing, the sun was shining and suddenly a car crashed into the tree."
"She phoned me while it was raining."
When you put when or while in the first clause, you have to use a comma (,). On the other hand, you do not need to put a comma before when or while, if you start the second clause.
When my mother broke the glass, I was listening to music.
I was listening to music when my mother broke the glass.
We can't talk about states with it due to its continuous form.