If you have integrated with one of the payment providers in the Setup > Integrations > Payment Providers section of WishList Member, there is an option to automatically create a Member account when someone purchases.
Typically, the user will pay and set their desired Username and Password.
By default, payment integrations will create an incomplete registration until the member completes the registration process. This allows the Member to choose their own login info which completes the integration.
But, if Auto-Created Accounts for Integrations is enabled, this feature will automatically create an account for the Member.
Creating the account immediately will result in the Member not needing to create their account.
The delay allows the member the chance to complete the registration process on their own during the amount of time configured.