There are many scenarios where 1 day insurance may be appropriate. One of the most common is making sure you are protected when borrowing a friends vehicle. Securing an extra policy for this could protect a no claims bonus built up if no claim has been made for a while. This could therefore be a cost effective option for more experienced drivers.
Temporary insurance also allows cover to be provided for additional drivers sharing the driving on holidays and other longer journeys.
Insuring an overseas guest is a scenario where taking out 2 day car insurance may be necessary as there own insurance may not cover them while they are abroad.