The purpose of the I/O module field wiring arm is to make troubleshooting and repairs easier for the technicians.
Wires are connected to a panel and land on a terminal strip. From the terminal strip, the wires are connected to the terminal assembly on the I/O modules.
Generally, all the inputs are grouped together (usually with a tie-wrap). Then, the inputs are separated from the outputs, which were also grouped together (also with a tie-wrap).
This gives the setup a clean appearance and aids in troubleshooting since the wires are well organized.