We have PI 7.11 (Dual Stack)
In NWA JCO RFC Provider there are a lot of entries which point in tab Server Configuration to PI Gateway and in tab Repository Configuration to any applicationserver of ABAP system in our landscape. SysNr, Client and user credentials belong to the abap system.
I can stopp and start the programs in NWA.JCO configuration.
In SMGW of PI I can see, that the programs are registered at PIs gateway.
If an ABAP-System, mentioned in Repository Configuration of JCO entry, is down, then the JCO server can't be started.
For what reason one would create such JCO entries?
In my oppinion gatway and applicationserver in a JCO entry should point to same system. Usually to PIs gateway and applicationserver.
Is there a reason why SAP spread the entries to different tabs?
As far as i understood your question you wanted to know why the JCO connection,JCO entry is necessary & why the entry for gateway and application server.
to answear your first question-
example- if user want to view payslip in EP there is an iview for that wherein the JCO connection name to your backend ECC system is mentioned in iview object details so if that JCO connection is not working he wont be able to view his salary when he clicks on the iview.
In summery-The JCO connection details contain the details of the system to which you need to connect.
and regarding the two different entry for message server and application server-
When you try to create a JCO connection for the first time it will ask if you would like to create the connection for load balancing or without load balancing.Accordingly if you choose with load balancing the Message server details entry becomes important and application server entry doesnot hold any relevance & vice versa for connection without load balancing.