Hi @Wojciech Jakubowski ,
Ok. Let me try to clarify the purpose of the portal sub resource -
If you are working from a VNet environment, like lets say you are using VMs in the Azure VNet as your development machines. If VNet is locked for any external access which is in many enterprise scenarios then users will not be able to access ADF’s UX. So, in that case to enable access to ADF portal user needs to create PE in their VNet.
Please let me know if any other questions.