why assign function module to process code? IDoc

Hello,

In inbound and outbound interfaces, we see function module is assigned to a process code. Why?

If the SAP design would have been to directly input function module in partner profile, for inbound and outbound; instead of process code, wouldn't the processing have worked? In other words, what is the reason for linking partner profile to process code and then process code to function module?

Thanks!

TW