The explanation as to why you need to do this is kind of complicated and goes off into the weeds, but I figured I'd clarify my previous post because I realised it didn't completely make sense unless you already knew what was going on:
Essentially, Mac OS ethernet drivers come in two parts, a generic .ENET driver 'shell' that is common to all Slot Manager-aware ethernet drivers, and then a device-specific 'enet' code resource that the .ENET driver looks up and loads, based on the Board ID field of the card's declaration ROM. The driver on Glenn's page is only the device-specific part of the equation, and even if your system has ethernet support installed, only later version of the .ENET driver support loading 'enet' resources from extensions such as that driver (older versions only looked for 'enet' resources in the System file).
By default, the Network Software Installer disk only installs or updates the .ENET driver and other ethernet support bits such as the Network control panel, if it detects an Apple-compatible ethernet card in the machine, hence the need to manually force the installation of an Apple driver to get those bits in the absence of the original vendor driver disk.
What I did with my driver disk (and from the driver disks I've examined, what I suspect the vendors did too), is take the Apple Network Software Installer disk, gut out all of Apple's drivers, and replace them with my own to give a 'one stop' installer that can bring a system up from nothing. Just the driver extension on its own isn't much use otherwise.
(and non-Slot Manager drivers such as for SE PDS or SCSI ethernet adapters are a whole different kind of headache, but that's a story for another time)