Little fires
Fire trucks, as we all know, are usually very long and large, with many attachments on them that they need to fight all types of fires. When they go down the street, racing toward an emergency, they take up a lot of room on the road and disrupt traffic when they try to get by.
Because Tokyo and other cities in Japan are so large and usually congested, they needed to make everything more compact, and this went from fire trucks as well. They have little fire trucks like these ones that get the job done, but just more efficiently. They look pretty cute as well, like large clown cars.