It would be very useful to be able to use templates to customize an entity. For example, I have a radiator thermostat that does not report its current temperature. As I have a temperature sensor in the room I’d like to use that for the current_temperature:
customize:
climate.living_room:
current_temperature: {{ states('sensor.living_room_temperature') }}
I can imagine there are plenty of other use cases. e.g. correcting sensor values when you know them to be off by a particular amount, changing icon based on state, etc.