What is ECU & ECM?
ECU stands for an engine control unit.
ECM stands for electronic control module which could be any of car modules such as body control airbag/abs/transmission/gateway etc.

Why we need cloning?
In modern vehicles there is several ecm linked together as a part of immobiliser. For example ford immobiliser information is stored in engine/body control/cluster and abs modules.
If one of the modules which store crypted security data got damaged, there is fewer options:
1. To find a good working set of all units which contain immobiliser data and replace them with new keys and locks.
2. To buy only the damaged ECM and then transfer data from original to new one, in other words to clone it.
It’s quite obvious that on the vast majority of cases the most economic solution is the cloning one.