For that extra grip length on your saber staff.
The 3 inch coupler is about twice the size of the standard coupler. It is great for adding a bit of extra length to the gripable area of a staff. The double vents in the coupler also make for a nice loud saber staff. It is notably louder then when I have my regular pommels on due to the larger vents and their placement around the sides instead of the bottom like on a pommel.