This accomplishes a number of things...

Pulls the filament off of the spools at a constant, low resistance around a gentle arc at a fixed position and angle. Without it, filament pulls off the side of the spool at an extreme tight angle causing the spool to bounce around during reversal and Z movement.

You don't need spool housings around the spool to keep the filament in place. Since it's being pulled off at a fixed angle, the filament no longer has a tendency to fall off the side and get tangled. It also doesn't unwrap when Z homes, the tube rises instead and the spool remains wrapped nicely tight. (which gets really bad when you stack a second spool on top)

Makes switching between the 2 spools really easy since when you pull one out of the extruder, the tube holds the filament in place instead of spinning out of the spool and making a mess.

Provides another place to stash your tools!

Print out just the bottom part if you only have a single spool. Screw it into the other square slot on top of your ToM. If you have two spools stacked, print the top part and snap it together, no screws needed, slide it up and down to adjust the height.

Note: you must use a tube to guide the filament around, otherwise it will twist around at a bad angle. A tube with an OD of about 6mm will press fit nicely into the small holes in the side. I suggest some PTFE tubing like this http://www.mcmaster.com/#52335k34 (note, that tube from McMaster might require you to make the holes in openscad slightly smaller) about 3 feet or so - it has to be long enough to form a nice gentle curve to reduce resistance.

You could also try mounting it on the back left replacing the cap on top of the left Z rod. That might allow for a nicer curve through tubing...