One might choose to do this because the admins thought they were "smart" and said, "Oh, I know, we will only allow 'known' MAC addresses to connect to the network! That will fix it."

And it does, kinda. Except it doesn't stop you from capturing clients on your "fake" network, and the goal isn't necessarily to be on the "real" network, the goal might be to just man-in-the-middle a juicy site or two, with is on the big Internet (say, the company's bank)

Capture the controller's login into the bank and win "free money"