Imagine you have a nice, new, cool, small, slim netbook (or notebook) which has no CD-DVD drive. So you need to buy one for 70 dollars or so.
But you can safe the money, if you have an older PC, which already has a CD-DVD drive. Just create a LAN (with a router or without). Then click on the CD-DVD drive, go to share options and share it (like a standard shared folder).
If you do this, you can use your shared CD-DVD drive to actually install GAMES or watch DVD movies on the netbook (notebook) which has no CD-DVD drive it self.