The 100m eithernet distance restriction used to be to ensure that if two machines tried to start talking on the same line, they would hear the noise from the other before they finished sending their packet, and could therefore know there was a transmission error.

That is, at least, what was described to me in Computer Networks I back in 2006. I'm sure things have changed as we've moved to always using full-duplex wiring and fully-switched networks.