Trauma Pads are designed to help reduce the impact to your body.
Unfortunatly Body Armor does not make us invincible, the impact of a round hitting your plates could break bones and cause internal bleeding just due to the sheer force (or more accuratly, the amount of energy that your plates need to stop). Trauma pads are designed to be another way to help disburse the impact.
Getting hit by a round can still knock you over, it can deform your plate to the point of broken ribs... long story short... it's not fun.
Here's a fun analogy...
You get puched in the somach... knocks the wind out of you right? Now, if you place a dense (perhaps Memory Foam) pillow infront of you the punch still lands and it might push you back, but it doesn't have the same impact... TRAUMA PAD... you dissipated the energy of the punch through the density of the pillow.
Trauma pads go in the same compartment as your plates (they belong closest to the body).
What Trauma Pads are not...
Technically, a Flak Jacket is Body Armor, however these "Jackets" were designed to protect the wearer from LOW Velocity projectiles... think flying debris and shell casings.
Sadly I have witnessed people selling Flak Jackets as "Bullet Proof" ... the problem lies with the velosity. Bullets by their very nature move at a high velocity. While it's true that some Flak Jackets have been demonstrated to defeat a bullet, this ability varies widely and is in direct correlation to the velocity and angle of the projectile, the distance it was fired and the weapon it was fired from.
It was claimed that the M-1951 flak jacket could stop a 90 grain 7.62×25mm Tokarev pistol round at the muzzle of the gun. However, even the Vietnam era revised flak jacket was not really designed to stop an AK-47 round (7.62×39mm) fired at close range. Nevertheless, it did a good job of stopping shell blasts, rocket fragments, and slow speed bullets fired at a moderate range.
Flak Jackets have their place and purpose HOWEVER, they are not "bullet proof"