The basic difference between a payment gateway and bill collection is the mechanic of “push” vs “pull”.
When a payment gateway is used for collecting the money from users, you need to “pull” them to an app or webpage that can support the transaction - this could be your website, your app, the payment gateway’s webpage, etc. Sometimes, these also require the user to create an additional account to complete the transaction or even do this behind the scenes without user consent.
When you use Collect BBPS, you are enabling users to search for outstanding bills and pay from within the comfort of payment apps that they already use. Users use their preferred apps, and can also avail popular cashbacks and discounts. Additionally, the payer apps also provide helpful nudges like reminders and push notifications to help you with your bill collections in a timely manner. In a way, you are “push”-ing the user to complete the transaction using their favorite channels / apps.
Finally, you don't incur the cost of maintaining your own payment gateway and customers can pay you 24x7 from any one of 400+ apps or 3.5 million agent locations across India.