A book suggests that we should not interchange "so" and "so that". "So" means "therefore", and "so that" means "in order that". However, it seems to me that in many cases they don't actually have a difference. For example:
Alice got up at 5:00 A.M., so that she could drive her son to school.
If I substitute "so that" with "so", this sentence presents the exact same causal relation to me:
Alice got up at 5:00 A.M., so she could drive her son to school.
I must have missed something here, since English is my second language. Could somebody explain the difference?