"Let's eat chicken for breakfast."
Translation:아침으로 닭고기를 먹읍시다.
I think it is because it is "for" breakfast. 아침에 actually means in the morning, so you could do other things at that time, but 아침(morning)식사(meal)로 or the shortened version 아침으로 actually means for breakfast. So 아침에 would mean "let's eat chicken in the morning", which is not the same. I don't know if that made any sense or helped in any way...