ACI Remote Leaf allows you to extend your ACI Fabric to a remote location through a IP Cloud. If you have locations that cannot justify a full fabric just connect up ACI leaves to the Rotuer on the remote location and connect up to the ACI Fabric through VXlan tunnels over the IP cloud. You don’t need to install spines, or APICs on that remote location. The leaves show up on APIC as part of the ACI Fabric and you can then use them just as if they were connected to your local Physical Fabric. If you cannot justify a full fabric on the remote side and do MultiPod (or Multisite), then this is a very good option.
Another very nice use I’ve seen of this is for a enterprise Fabric on one floor of a high rise. and then other floors have small secure data centers. You connect up routers on the main floor and then through the raiser connect cables to the other floor / floors and then hang off the Remote Leaves from there. This video will guide you step by step on how to install Remote Leaf. You can try it out in your lab and see how you like it.