They are new in many areas and operate in parallel with the existing bins.
Many people have received extra caddies because I think there is now a mandatory weekly food waste collection, and you are not supposed to put your food waste in with the garden waste.
The motivation behind this is probably not to leave food waste around any longer and reduce the amount of food waste people put into the landfill bin.
Fro me it looks largely a waste of time. For food waste I used to put in with the garden waste, never in the landfill. It is a bi weekly collection, so if I had something I do not want to rot for 2 weeks in the bin I would either bag it up in compostable bags or bag it up in compostable bags, seal it and freeze it until it can be thrown away.
The new caddies I don't get because they will cost more to collect, are far bigger than people need (we have 2x, one is massive - I always though the idea was to actually eat the food not throw it away but ho hum).
They would have been better off changing the cardboard collection (e have no specific caddy here so when the wind blows the cardboard goes all over the place) or separating out the existing plastic metal.
Unfortunately it is government mandated I think, so the local councils have no real choice on it.