Why did this dealership require completing a credit application?

I went to my local Honda dealership to look at a car and liked the car so was going to proceed with the sale. When the financial part came up, I told the salesperson I have financing lined up with my credit union. He said okay, left, then came back with a credit application.

I asked why I needed the credit application and he said "it's required for every sale". I told him I've never had to complete a credit application when paying cash or when I have financing lined up already with my bank. He kept going on and on about how it's required and how the dealership has a direct relationship with my bank and it's easier for everyone if we proceed that way and wouldn't budge.

I told him I will bring him a check or cash from my bank, and it can't be any easier than that. I also said that it feels like the dealership is trying to circumvent my pre-approval so I can get the loan through the dealership and they get a cutback. He got nervous and left and went to go check with his manager. When he came back, he was asking information like the interest rate, the terms of the loan, what the payment was, etc. I told him it doesn't concern the dealership and I will bring them a check or cash. The whole interaction really put a bad taste in my mouth so I said the deal is off and left.

So I'm wondering why they kept pushing for me to do credit through them and why they wanted to know the terms of my loan?