One of the main rules for using the past progressive or continuous (was + gerund) is when the word when appears.
- to be doing something when something else occurred.
Most grammars will explain it that way.
The first action was ongoing WHEN the second action occurred. Even if the word when is not specifically mentioned, it can be implied.
- I was eating dinner around 7 o'clock. That describes what you were doing at that time.
There are three basic uses (see that page from englishpage.com for many examples)
- Interrupted action
- Specific time as an interruption
- Parallel actions
- Creating atmosphere
She walked down the street when she met Steve. is fine but means:
She walked down the street as the result of meeting or seeing him.
She left the room when she saw him.
She paid her bills when they arrived in the mail.