By default in odoo, when you fill sign up form and then odoo create portal user inside system, but for enterprise system or some application, they charge based on user services so it is not suitable to create user in just filling form but we have to authenticate user request that this sign up requst is generic or not and based on that we have to create user.

So for this purpose, the Website Portal User Request application allows you these all features like manage user signup request and authenticate that request. In that when user sign up then the request is generated as a lead in CRM module and from this, sales team authenticate request and based on that perform appropriate action like accept signup request or refuse. If accept then they need to create portal user based on given detail.