The 28 mm Sawyer Coupling has several uses, I use it to back flush my Sawyer Squeeze, I use it to transfer water from one bag or bottle to the other, I use it for filtering from my dirty bag to my clean water bag. I use it so much I keep an extra as a back up so when you order don't get just one or you'll find yourself reordering another.
I would recommend getting one. I use it with a sawyer filters, just screw this to the filter and the filter to the vecto and it makes filtering water super easy. I fill my 1 liter bottle, and fill my hydration bladder with the bottle, and this adapter makes it so easy.
These couplings are essential for back flushing and setting up other systems with Sawyer filters. I only needed one, but I could see easily losing one, so I picked up two.