Yes there's a reason.
The installer won't allow the install without it. I guess it does a quick hardware check to see what type of en/sc is connected. Perhaps there is a slight difference and it configures settings slightly differently or something depending upon which type you have? In any case, it absolutely refuses to install without the hardware being plugged in and recognized by the installer.
And I'm guessing it's unstable and freezes before the driver is installed because apple didn't predict ethernet via a scsi port. I could be wrong here but I think the scsi proocol has a type flag or something that identifies the kind of item it is - ex: scanner, cd-rom drive, hard drive, etc... And there is a "unknown" option that is there for anything that doesn't fit... I'm guessing that the en/sc is set as an "unknown" device, and the mac's additional extensions (maybe tcp/ip or opentransport) don't know what to do with it so it freezes. Just a guess of course. You'd probably have a better idea than I as of why that is so.
why wouldnt you install the driver before you hook it up so it wouldnt freeze the machine? any special reason for that step?