There are many scenarios where 1 day car insurance may be needed. One of the most common is making sure you are protected when borrowing a friends vehicle. Taking out an extra policy for this could protect any no claims bonus acquired. This could be a good option for careful drivers.
Another reason temp motor insurance is taken out is to provide insurance for an additional driver so driving can be shared on a longer trip.
Protecting a visitor from abroad is another reason why taking out short term insurance may be necessary. As is being covered for 24 hours when needing to drive a new vehicle home from a dealership.