Ddm 4000 midi thru

just wondering exactly what this is suppost to do, a normal midi thru takes a signal in and splits it, for example using midi to control more than 1 sequencer, but this makes no sense at it takes 1 in and would just be spitting it back out, if someone could shed some light on this it would be great, is it that the thru takes a signal in and puts it back out? or what, i have no idea, someone who can give a good answer and back it up would be greatly appreciated, thanks

I'm thinking about picking one of these up how do you like it? have you had any problems with yours?