So coming to Idea of why do we need this. Actually in old days and galaxy far far away everyone was running P2P circuits. As you might be aware on those P2P links we use to run PPP. Though Cisco was cool enough and came up with HDLC.
But the problem was that there was no Native QOS Mechanism supported by PPP or HDLC by own. And of-course in those days we didn't have QOS based on MQC.
So one day one evil engineer came up with idea "Alice - Hey Bob ! I know Frame-Relay supports a native QOS mechanism called FRTS(Frame-Relay Traffic Shaping) so why don't we use to apply QOS on our P2P links? "
But Frame-Relay on Point to Point Circuits ? How we gonna do that ? What we gonna name this thing ?
Hmmm... We call this Back To Back Frame Relay :-)
After this cool story now lets take a look at our simple topology and configuration we require.
We disabled Keep Alive in this implementation since there is no Frame-Relay Switch involved in topology(or in P2P links). Also on both sides we must use the same DLCI number.
So now you know that Back To Back FR was only method in existence in old days to implement QOS on P2P links for stuff like VoIP etc. All you gotta do is to create "map classes" to apply FRTS on these links.