There would be many situations where you would convert from fiber to copper. FYI saying fiber to Ethernet is not technically correct because fiber also uses Ethernet.
Ethernet is a standard means of transmitting data. Specifically 802.3 .
With that said when have you ever seen a computer with a Fiber Network Card? So at some point in the network the Fiber has to be converted over to copper.
Also Fiber equipment is very expensive. To keep costs down while enjoying the speed benefits of fiber you would only use fiber at the points in your network that you would identify as bottle necks. Mainly the core and distribution layers.
And further more, 100mb is (in today's standards) pretty low speed for a network. Pretty much every node on the LAN should be running at gigabit speed. So with everyone running at gigabit speeds having fiber on one end is a great way to deal with bottle neck issues.
Lastly, you should go and watch a CBT nugget video or an INE video. In fact go and watch any other technical training video on the net. You will see that having one of the host inserting jokes here and there makes a huge difference in information retention. The video series I mentioned all take a very technical and monotone approach to IT training. After 10 minutes of listening to the host it's very difficult to stay focused. What I like about IT pro TV is that wise cracks always brings you back in and doesn't let your brain doze off. So Nate keep the lame jokes coming.