When you recharge a voucher when existing plan is active, Jio will queue it and make it available once your existing plan gets over. For example, if I recharge for 303 and 503 together in Jio, it gets queues up and can use one after the other/
However, other providers will kill your current plan and activate the new plan effective the date of recharge. This means part of your old plan is lost. Other providers provide good offers mostly before the expiry of your current offer to get benefited from this lose.
For example I was in 84 days validity plan in Idea and I got another 84 day validity offer of 397 much ahead of validity expiration of the old one. I confirmed with Idea that if I recharge the new plan I will lose benefits of old plan for the remainder of the offer.
So effectively I am not getting the full 84 days of my old plan. Will the refund? Of course no. So who gains? You can guess without any doubt.