Almost free 2-port and 4-port fanouts, IEEE 802.3 compliant, Ethernet Version 1.0 and 2.0 compatible AUIs

I don't know either, but I might hazard a guess that they originated in the days of thick ethernet, when taps were expensive, and had to be fitted at specific intervals on the thick coax. Then perhaps it might make sense to connect two devices to one tap/transceiver. I'm just guessing, though. Now if they'd been the other "gender", so to speak -- ie, if they could be used to connect one device to two transceivers -- it would be tantamount to a bridge. Then I could probably use one as a media converter by adding both a 10base2 and a 10baseT transceiver, and link the 10base2 part of my home network to the 10baseT part without leaving a power-hungry repeater running all the time. I don't think they'll do that though. I once tried it with two transceivers back to back, and just got millions of collisions.