I don't really get longtails, can someone explain?

You get a nice box in the back or whatever but it's not really THAT much bigger than a box you could fit on any old regular bicycle

You can plop like two children there but then you can do that with Long Johns and 3-wheelers and such, too and ALSO you have the fuckoff-big box. They don't really seem all that shorter than comparable versions either.

Anyone got one or know why someone got one?