Computers could never replace teachers.

The things which computers cannot teach

In my opinion computers cannot teach how to listening  to the people. I know the computers give us a lot material for understanding people but don't teach how. Furthermore the computer cannot correct you when you speak wrong  also when you make a conversation with computers cannot answer  you and you don't see the reactions when you are talking.
In fact you cannot have a real relationship with computers as you have with a person. The teachers can understand every situation about your feelings and the most important thing for my is the computers always need someone to tell what it  need to do.

The subjects which computers are most useful for.

I think computers helps a lot in maths, it have a lot of programs about maths for each year. in fact my child used everyday for practice. for example one day his practice algebra , next day fraction and another day geometry and so on.
Personally in the subject that the computer help more is in math.