If you buy a fare-ticket and fail to use it, there is room thus made available for someone else; but if the place were secured to you it would remain vacant, and yet your ticket would secure you another place when you were presently ready to travel.
However, no explanation of such a system can make it seem quite rational to a person who has been used to a more rational system.

If our people had the arranging of it, we should charge extra for securing the place, and then the road would suffer no loss if the purchaser did not occupy it.
The present system encourages good manners--and also discourages them.
If a young girl has a lower berth and an elderly lady comes in, it is usual for the girl to offer her place to this late comer; and it is usual for the late comer to thank her courteously and take it.

But the thing happens differently sometimes.

When we were ready to leave Bombay my daughter's satchels were holding possession of her berth--a lower one.
At the last moment, a middle-aged American lady swarmed into the compartment, followed by native porters laden with her baggage.

She was growling and snarling and scolding, and trying to make herself phenomenally disagreeable; and succeeding.