As dunczn says, they serve different purposes. And they have different quality, too. The one on the back is meant to make high quality photos and video (8 Mp in my tablet) and it needs to be on the back, otherwise you couldn't use the display as a viewfinder. The one on the display side is a simple webcam, for using Skype and such. That is filming you while you look at the display. It all makes sense... :-)
The two cameras have different purposes. The back camera is meant to take photos of people, animals, objects, scenes and the like, but the front camera is supposed to be used when you want to take pictures of yourself. This front camera is also used when you are video calling.