The death certificate is a certified copy of what is written in the death register. The registrar can let you have a death certificate if you want one, but you will have to pay a fee.
You will need a death certificate if you need to apply for Probate (see page 30), and for any pension claims, insurance policies, savings certificates and premium bonds.
You may want to ask for more than one copy of the death certificate straight away, as the price goes up if you need one later on. The registrar may not be able to give you all the copies straight away. They may ask you to call back or ask you to pay an amount towards postage so that they can send them to you.