The auxiliary group has space for two items: one active and one passive.
The active items are manually controlled by the player and include things such as instant refill of all your nitrous bottles for a quick double burst on the finish straight or a repair kit to get some quick health back. This health can be hugely beneficial when you find yourself in a bit of trouble and are in need of getting back to the garage while limiting the risk of getting wrecked. (Need to protect that heat multiplier at all costs.)
The passive items, on the other hand, are active all the time as long as they are installed on the car. This category includes things like re-inflatable tires which reduce the time it takes before the tires reinflate after hitting a spike strip, damage increase so that the cop vehicles can be taken out faster, and various items that give increased nitrous gain from activities like jumping and drafting.