It was decided to employ him after a rat was seen scampering across the front door in full view of the cameras!
There have been many "Chief Mousers to the Cabinet Office". Before Larry there was Humphrey. The home of the Prime Minister was vetted as suitable before Larry was allowed to live there! Great stuff.
It is nice that a rescue cat was employed. And it is nice that a cat is showing his worth as a working animal. Many people see the dog as a working animal and the cat as an ornament. This is not necessarily the case. The wild cat was first domesticated for his mousing skills. The Maine Coon was first a semi-feral farm cat. These are the origins of the domestic cat; very much a utility farm animal.