And entrapment. Many users are afraid to hit the back button when they're in the middle of submitting information. This design leverages Back Button Anxiety. It forces the shopper to make a decision. It may not alter the minds of anyone set on quitting or going forward, but it will most certainly influence anyone having second thoughts or thinking of coming back later. Even if it's just to look at an item description one more time, if that option carries risk of sunk costs, they might just go with it. But chances are they were about to get distracted and end up not buying anything, so the shop wins.
I find the close button is often a satisfying alternative if a site won't let me go back.
I don't know how much money Amazon have lost from me in recent years because their order pages are so awkward that I've given up and bought elsewhere instead, but it must be the equivalent of several hundred dollars by now, if not thousands.
Amazon is a perfect example actually... I was trying to find a reference but gave up. I remember reading about the logic behind isolating the checkout experience. For a while I don't think they even had a link to go back to your cart. Even now they make it non-obvious and have a dialog that will ask you before you leave. They also remove links of any products to their product pages. All trivial UI elements you'd think would be good design, but it turns out entrapment is better. Heck, Amazon would know.
They do have an accordion, but they minimize data entry to an absolute minimum. It's all multiple choice and submit.
From personal experience, streamlining a checkout process as much as possible can make a rather dramatic difference to conversion rates. However, the "as possible" is important. You don't want any unnecessary distractions, but if you back someone into a corner so they can't check something they are worried about or correct a mistake, they'll probably just go somewhere else.
And entrapment. Many users are afraid to hit the back button when they're in the middle of submitting information. This design leverages Back Button Anxiety. It forces the shopper to make a decision. It may not alter the minds of anyone set on quitting or going forward, but it will most certainly influence anyone having second thoughts or thinking of coming back later. Even if it's just to look at an item description one more time, if that option carries risk of sunk costs, they might just go with it. But chances are they were about to get distracted and end up not buying anything, so the shop wins.