Good question, Majo0od!
Member Exclusivity is a business strategy to gain more members who have signed up for the service and is a marketing ploy to get more signups/subscribers and get ample emails and numbers to push their marketing to.
Whereas a website without mandatory sign-up aims to deliver a smooth experience to first time users, after which they can pitch them to become a member.
Both are honestly good business decisions and the first scenario can help the user in the long run.
Website A offers to purchase a product without sign-up.
Website B has a mandatory sign-up procedure.
User logs on to Website A and decides to purchase a product he likes.
He checks out and now is asked for all his details including address, telephone number, credit card, etc. He enters all of those and checks out with the product.
User logs on to Website B and isn't allowed to buy a product unless he signs up. He signs up and now clicks on Purchase. In the cart, he already has his details input and only needs to confirm the order.
Website A allows a smooth onboarding process. Con: Requires to enter details each time.
Website B allows a smooth purchase flow. Con: User might purchase on another website.
The benefits of Website B are that it allows them to interact more with the users as a business themselves as members than they could with the Website A strategy. Also, this means that since the User is a member, they are bound to visit again due to the perks of membership, and the website can contact them through emails or texts due to registrations to attract them to offers.
The correct strategy, both in case of business strategy and user experience to me is to not restrict purchase to membership only but to encourage purchase without a membership and once the purchase is made, show benefits of being a member on the confirmation page.