It’s for doing src/dstnat with router’s own traffic. It wasn’t available in old versions. When it’s forwarded traffic, you have:
prerouting/dstnat → forward → postrouting/srcnat
But for router’s own traffic (to/from router) you have:
prerouting/dstnat → input
output → postrouting/srcnat
So you couldn’t do dstnat for outgoing connections (send it somewhere else) or srcnat for incoming connections (set different source). With these two chains you can.
Some examples: http://forum.mikrotik.com/t/dstnat-in-output-chain/104694/16