Regarding what Christian said, here is the documentation:
Be aware that the documentation for Forms/Reports Release 1 and Release 2 is the same for managing OHS and the other core components.
I suspect what you are actually asking is why did Oracle choose port 8888 for the OHS listener in this FMw installation. There really is no specific reason for choosing 8888. It could have just as easily been 1234 or 9876. The reason we don't use port 80 (or 443) is that there are restrictions on Unix plaforms when using ports below 1024. Using a larger value makes installation and administration easier during the initial setup. The port number has nothing to do with load balancing. Load balancing is something you configure on your own.