Switches are unlike hubs or repeaters. A hub simply rebroadcasts every signal on every port to every other port, which (while inefficient and slow) is easy to create. A switch, on the other hand, intelligently directs traffic between systems by routing packets only to their proper destination. To do this, it keeps track of the MAC addresses of the NICs plugged into each port. MAC addresses need to be unique or at least highly unlikely to be repeated for this to work, which is why manually setting a MAC address can have unexpected consequences in a switched network.
This was first published in March 2003