Sometimes one past action interrupts another.  We use the Past Continuous for the longer action or the action which was already happening and the Past Simple for the shorter 'interrupting' action.
Examples:
     It was snowing when I left home this morning.
     I was working in the garden when I heard a loud noise.

Note that we can only use when (not while) before the Past Simple.  However, we can use when or while before the Past Continuous.
Examples:
     When I met John, we were working for the same company. (when = at the time)
     I met John when/while we were working for the same company. (when = during the time)