Compare:
- Вчера я шел в магазин купить хлеб.
- Вчера я пошел в магазин купить хлеб.
If you translate the first as "Yesterday I was going to the store to buy bread," you need a further clause in English such as "..and I feel down." If such a clause is not necessary in Russian what is a possible translation that makes sense in English? Natasha's site once had this: Вчера я шел в магазин. Я купил .... (I forget what it was.) Why not "я пошел?"