A grant of probate is not always required, for example, if the deceased had no assets or a small amount of money in a bank account.
However, it will always be required if the deceased owned property in their sole name, and sometimes where a property is owned jointly as tenants-in-common (where each joint owner owns a separate share in the property) or if the deceased owned a substantial number of shares or had substantial savings.
It is important to remember that an executor is appointed in the will and their authority to deal with the estate comes from the will. The grant of probate confirms that authority.