[I think it] Lets you make multiple prints on one piece of paper. You may want to print the same neg multiple times, or different negs on the one piece of paper (less useful IMO).
I have one called a Durst Commask that lets you do 1 8x10 or 2 5x8 (or 5x7 with an additional divider) or 4 4x5. All sizes include the border you get (the image area is slightly less). The one you have seems to let you do more combinations. I rarely use mine with the smaller sizes, but use the 8x10 regularly as their are no borders to adjust, you get what it gives.