Benefits of having a Shopping Cart (as opposed to "straight to checkout")

I have a client whose colleagues are pursuing an eCommerce Out of the Box product that doesn't have a dedicated shopping cart screen. Their product works such that when you want to buy something, it goes straight to a full checkout form.

"How can the user add more items then?" you might ask - the product lets the user go back to the catalog and "add" more things, but there's still no Cart page. Even the menu label they have is "Checkout" (rather than Cart + icon)

My client and I are averse to this solution knowing the psychological comforts and conventions in having the security blanket of a cart, but I'm struggling to find secondary research to demonstrate that not having a cart is bad, and/or there are measurable benefits to specifically having a discrete shopping cart page before checkout commitment.