We're almost at that point where every site offers this, but one of the biggest barriers to checkout completion is asking your users to sign up or create an account before checking out. It adds that extra bit of hassle and thought to someone's journey: "Do I need to? Shall I just go to another site I'm already registered with?". Asking for passwords is getting in the way, when at this point you should be getting out of the way and making it easy for them to part with their cash.
Even if you're a site where the user will need an account to log back in later—like an accommodation or transport provider—you still don't have to get the user to create a password up front. And that's the only field difference for new members: a password. You can get them to create it at the end of the process, when their order is placed, or you can create them an account with their email address and email them a password reset later.
Be smart about it: the user didn't come to register for your site, they came to buy your product or service. Give them that before asking for more.