If it is a puppy or young dog you get free insurance for six weeks. Also if it is a breed that has health problems of any sort and you are going to have it health tested (i.e. eye test, hearing test, heart test, hip scored, dna tests) then you need to have KC registration and also it will help the breed by having the information published.
It only costs £15 to transfer to your name, and the certificate is quite smart so for how little it costs I would do it, but there is actually no real need. Registration in your name is not proof of ownership, but I am proud of the dogs I have bred and like the owners to have them transferred. Having said that, if you check the records, I still 'own' quite a few of the dogs I bred, even though I sold them years ago! Don't be fooled by people who offer a cheaper puppy without registration (it often means they are not breeding from registered parents and they are probably not health tested).