I’m afraid that I don’t know exactly why it works! As to what it’s doing, you can see the PXELINUX documentation at https://www.syslinux.org/wiki/index.php?title=Doc/syslinux, specifically the section about the configuration file. Briefly, it’s adding an option to the kernel boot parameters to tell initrd which network interface was used to boot the system.
I’m afraid that I don’t know exactly why it works! As to what it’s doing, you can see the PXELINUX documentation at https://www.syslinux.org/wiki/index.php?title=Doc/syslinux, specifically the section about the configuration file. Briefly, it’s adding an option to the kernel boot parameters to tell initrd which network interface was used to boot the system.