Benno knows about magic and the Devouring. So it might not be that strange to have a little child speaking on equal footing with him.
He also immediatly understood the value there's in Main with the Kanzashi and Shampoo, because it seems some of his business(es) deal with clothing/apparel.
So when she talks about a new and cheaper way to get paper in a world where it is a rarity for nobles, of course he listens...
Also, let's be cynical: Main dying quick is his way out the contract. It's not a surprise he wants the first batches of paper right before her baptism as he knows it might be her time limit.
Although we know she won't die and will meet that priest who is probably key to her survival... And Benno might be the one helping her getting proper help because it also serves his interrests for the future when he understands the true depths of Main knowledge and the large number of new markets he can create thanks to her.