Hexagonal Architecture: What is the purpose of the input ports?

Pretty simple question, but googling brought me nowhere: What is the purpose of input ports in the hexagonal architecture?

We are doing Java and seem to have some misunderstandings regarding the input part of this pattern. We presume:

• Domain Core / Business Logic: This part of the architecture describes our business and should be free of any framework specific / technical knowledge. It should be easily moveable from one technical infrastructure to another.
• Outbound Ports: Defined by the core and implemented by the outbound adapter this is classical dependency inversion principle, so that the core stays free of any adapter-internal / technical details.

But what is the purpose of the incoming port? Every article on the web mentions it but they are all lacking an explanation for its purpose. Why should I care about defining this port and not let the input adapter access my business logic "directly"? Why this abstraction?