An enclosed checkout process takes users away from the distractions of your regular website. By removing navigation elements, and extraneous headers and footers, you can create a focused, stripped-down checkout process that forces your visitor to focus on the task at hand – buying your product.
Crucially though, don’t go too far with this tactic. It’s important to keep your checkout process as a recognizable part of your website, and offer visitors a way to go back a step. Sending a user to a separate domain can also cause serious trust issues, and lead visitors to worry their browser session has been hijacked.