Some NDs and some Fiats have a ride height sensor at the rear of the car that allows the headlights to compensate for a heavy load in the trunk. However, if you lower your car, this sensor will get confused and will aim your headlights at the ground. This is obviously a bad thing.
The sensor is there to ensure that the headlights don't point to the sky when you have a load in the trunk. Since it only measures the rear ride height, it assumes that a lower rear ride height equates to a low tail and a high nose. It compensates by aiming the headlights lower. It can't tell if you've lowered the front of the car as well, it's working on the assumption that it knows the stock front ride height.
If you drop the car without addressing the leveler, your headlights will be aimed low and closer to the car than is ideal. They'll still work, but you'll have decreased night vision as a result. A 1" drop will be enough to reveal this. If you mostly drive in well-lit areas, you may never notice.
This part allows you to adjust the length of the arm between the sensor and the control arm and compensate for the lowered ride height. The car will then know what the new normal height is, so your headlight adjustment will continue to work as intended. You'll be able to see at night, and the headlights will adjust depending on the load on the car. Everybody wins!
Not all Fiat models have the rear sensor. It is not present on Classicas. As far as we know, all US 2016-17 Miatas have the sensor but it is not present on all 2018 and later models. To be sure, have a look at where the rear sway bar attaches to the suspension on the left side of the car. If there is a small lever attached to an arm beside the end link, that's the sensor.
Simple installation. Includes everything required. These are available elsewhere with magic JDM dust sprinkles for $70.