Uber India — why the "credit" in my account

Address:

New Delhi, Delhi

I don't understand why am i supposed to pay any credit. I know the day it registered in my account and that was because your driver didn't pick me up and cancelled my ride while it was my university examination day.
And after that when i used uber for another ride, of course i was suppose to pay the credit amount too, and yes i did, even though the fare for my ride was showing near zero inr.
Still why is there a credit that i've already paid!
And also i receive promo code sms but can't ever use it, ever!!