Question

why did father want swami alone in the office room

Answer 1

Swami’s father challenged swami that he can’t sleep alone in this case swami slept alone without sleeping with his grandma

Answer 2

Swami was ordered by his father to sleep alone in his office so as to fight against the fear of darkness.  Swami had the habit of sleeping with his granny which his father didn't like. He wanted his son to be brave and become a courageous person in his life. Swami's father wanted to test his courage and so he ordered him to sleep in the office alone one night.