Sub Boxes that have a volume bigger than 30 litres will have to be braced. A box smaller than that will not have large panel areas that will flex.
Bracing is good for 2 things, it dampens the standing waves inside the box and it dampens the resonating speaker box walls.
Personally, I noticed a huge difference in sound quality and pressure inside the car (Slot ported box with 8 x 250 RMS 10" subs) once I braced the box to the body of the car with thread bar - bolted from underneath the car and then I also braced the inside of the box with 4 x 1" Dowel Rods. Once the box is no longer flexing and moving around in the boot, it makes a huge difference. Every DB counts and I managed a gain roughly an extra 1.7DBs from reinforcing my box and bracing it properly. I would highly recommend bracing your sub box if you have any large panel areas.
A cheap and easy upgrade that works!!!