There are numerous scenarios where 1 day car insurance may be required. One of the most common is making sure you are protected when borrowing a friends vehicle. Securing an additional policy for this could protect a no claims bonus built up. This could therefore be an attractive option for drivers who have built up a big no claims bonus.
Another reason temp cover is taken out is to provide protection for a driver sharing the driving on a longer journey.
Insuring a foreign guest while they are visiting is popular reason. As is needing short term cover when taking a newly purchased vehicle home. Taking a test drive and requiring cover for a day can be another scenario.