Toyotoyo makes a valid point.
Also though, they are handy for when you are bored of doing the standard races in LFS. There's not really a big selection of cars or tracks, and likely there are tracks you don't race with certain car combinations, and certain cars you never race at all.
Races where the winner each round is forced to add a passenger can help add another way to influence results, and is immediately obvious to everyone else that the weight handicap imposed upon the winner has or hasn't been made.
I also doubt that the addition of the passengers took anything like as much work as modeling and creating the physics for a whole new vehicle. Maybe about the same amount of time required for you to be able to open your trunk and reveal some speakers that are there for everyone but the passengers enjoyment, along with usual retarded elements like playstation and dvd screen etc.
"Oh, hang on fellow racers, how about I pull over, pop the trunk and we all play Tekken with 6 Gigawatts of sound? I know it's cold and everything, but the speakers are surrounded in neon too..."