It's like a peer-to-peer courier booking system. They connect bluedart (courier) and the merchant (shopping sites).
Since shiprocket is heavily invested in tech, the merchants can handle orders with a click of a button. And they handle end to end booking and tracking. Also bluedart and other couriers may not have mature tech. So integration for merchants would be painful.
Kinda like how Amazon pay EMI works. You choose amazon pay emi but the loan gets placed to its partner bank. Amazon acts as a peer to peer lending network between customer and bank. Sonething like that.