Both Hybris and AEM can build a website independently. Still why do we integrate them like Hybris PIM and AEM? What is the advantage of doing so?
Hybris is an eCommerce platform. You build your web store on top of it. It is quite a common practice to integrate different platforms for separation of concerns. In this case, you can have your Storage Management or Customer Relationship Management, developed in AEM and your Web Store in Hybris because Hybris is specialized in eCommerce and AEM is specialized in CRM, ERP and others.
One more note: PIM in Hybris stands for the Product section.