The past continuous stresses that the action in the past had some duration.
Examples
A: What were you doing at 4 p.m. yesterday? B: I was playing tennis.
John was working all evening yesterday.
The past continuous often goes together with the past simple. The past continuous shows a longer background action whereas the past simple is used for the shorter action:
I was doing something when something happened.
Examples
I was having a shower when you rang the doorbell. was having — long background action, rang — short action
Helen was cooking — longer background action dinner when the children came back. was cooking — long background action, came back — short action
It wasn't raining when I woke up. The sun was shining and the birds were singing. wasn't raining, was shining, were singing — long background action, woke up — short action