What is the name for the piece of equipment that hooks to the tractor, then the 3-point planter hooks behind it.

I am not sure if the unknown named piece is 3-point or tongue pulled.  The piece has a large tank in the middle of it, and as many coulters as the planter has rows.  I want to just call it a 'coulter cart', but don't think that's the name.  It serves to fertilize right in front of the planter.  The one I saw folded, 90*, and the planter stack-folded. 

What would be the advantage of this piece of equipment, instead of just pulling a tank behind your planter?

Sorry if it sounds like a dumb question...I just have not seen anything like it around here, and was wondering it's purpose..and name...