Past Continuous can be used to describe scene background or the scene itself. We need it to describe the atmosphere at a particular time in the past, in a scene that was unfolding over a period of time. Usually, in the beginning of a story, but not necessarily.
The sun was shining, and the birds were singing, everything did seem okay to them. (scene background)
The man was standing here, then he walked over to that window. (scene itself)
1. Action was taking place in the past.
2. Action was incomplete (continuous).
3. The scene background or the scene itself was unfolding over a period of time.
Other ways to describe a story in the past
For instantaneous actions in the past, we can use Present Simple. Yes, this is true, no matter how paradoxical it may sound.
As you can notice, Continuous line of tenses is used to express ongoing action. This time, such an action takes place in the past during a period of time a story was unfolding.
Also on Past Continuous: