If a property e.g. a home is owned jointly as joint tenants this will pass automatically on the death to the surviving joint owner. All bank accounts which are held in joint names again pass automatically to the survivor. It may be that on the death of a Wife/Husband/Civil Partner probate is not required.
If a property is held in joint names as tenants in common that property passes by way of the Will of the deceased or under the rules of intestacy and therefore a Grant of Probate or Letters of Administration would be required. It is likely that a Grant or Letters will be required if bank accounts/ISA’s /shares etc. held in the sole name of the deceased.
On the death of the person it is the duty of the Executor to administer the estate of the deceased i.e. to collect in the assets, pay liabilities including inheritance tax ( if payable) and distribute the estate in accordance with the Will. The Executors may not be willing or able to act and in this situation the law sets out who can act as Administrator with Will annexed. If someone dies without a Will again the law specifies who can act and who is /are the beneficiaries.