-действие происходило в определенный момент в прошлом:
At 8 o'сlock last night I was talking on the phone.
-для описания действия, которое происходило в прошлом и было прервано другим действием (Past Simple):
They were playing footaball when their mother called them.
-два и более одновременных развивающихся действий в прошлом:
He was reading while his brother was watching TV.
-для описания атмосферы, обстановки, описание природы:
The birds were singing and the sun was shining. I was sitting in the garden...

​

​Past Continuous:

Past Continuous vs Past Simple