AFAIK, when the last central govt took over, they improved the system of reservations to allow operational upgrades. I have not been able to find out much about how it worked other than that like airlines, if they oversold 1 class, they might bump you up a notch to the next level.
Some people may not want to change the type of service.
If you are used to a 3 tier system with plenty of people in the carriage then the concept being 1 of 4 people in a locked room is worrisome to some.
I think this is an effort to only upgrade those who wish to be upgraded.
YMMV.
Thanks. With two of us traveling, my only worry would be that one might get upgraded and the other one not. So I'll choose this option only if there is a way to specify that we want to travel together, in whichever class.