The reason you run a richer mixture is for a variety of reasons. The first is how combustion takes place in a nitrous motor is different than in a NA motor. Nitrous oxide does not burn, it is an oxidizer. It provides more oxygen, so more fuel can be burned, and the result is more power. The atoms in a nitrous oxide molecule are bonded together. At 565 degrees F, the bond is broken and the oxygen is then free. By adding nitrous oxide to an engine, the total amount of oxygen is increased and other gasses that do not support combustion (mostly nitrogen) are decreased. This speeds the burn rate and requires less timing advance for peak output. It is hard for many people to grasp gaining power with less timing, but it’s a fact. Peak cylinder pressure must occur approximately 20 degrees ATDC to make peak power. If you speed the burn rate, peak cylinder pressure will occur to soon. It is easy to run too much ignition advance with nitrous, but too much will not only hurt power, it can quickly bring a nitrous engine into detonation and destroy it.
This brings us to another reason you need more fuel in the cylinder. To keep the engine out of detonation, you must control the extra heat that nitrous makes. The easiest way to do this is to add more fuel. All nitrous systems come with rich jetting to give you a safe starting point. The extra fuel takes away heat and raises the detonation limit. If you don’t try to over do it, and keep the hp levels within reason, running slightly richer should be all you’ll need to control detonation. Running richer will both reduce the power output, but will raise the detonation limit to allow more nitrous to be used .
And last but not least "Atomization". You need a richer mixture to better the chances of the nitrous mixing with fuel. If a nitrous engine runs lean, it can destroy the engine in a matter of seconds. There must be enough fuel for the nitrous to react with, if there isn’t, temperatures rise rapidly. The oxygen that couldn’t react with fuel will oxidize any parts that get hot enough, and the next thing in line to burn is aluminum.
