One can think of numerous reasons where drivers may take out a 1 day car insurance policy. One of the most common is making sure you are protected when borrowing a friends vehicle. Securing an additional policy for this could protect any no claims bonus acquired. This could be a good option for careful drivers.
Another reason temp insurance is taken out is to provide protection for an additional driver so driving can be shared on a longer journey.
Test drive car insurance is a popular reason people take out temp cover. As is requiring 24 hour cover when driving home a recently purchased new or 2nd hand motor.