When a guest customer (not logged in user) tries to checkout WooCommerce will render Create an account? checkbox.
By default this checkbox is not checked. If you want it to be checked by default you need to hook into this filter
Add this to your theme's functions.php or a custom plugin you may have.
I think it's a good idea from usability standpoint and also from business standpoint. You can later calculate how much revenue a given customer has brought in.
__return_true is a "constant" in the context of WordPress. That way we don't have to create a function whose only purpose is to return true/false or an empty string.