I was looking under my car for one reason or anything (noticed this a few weeks ago but kept forgetting to ask about it) and noticed I was missing a plastic piece on the bottom of my left rear shock. Here are pictures I took underneath from the rear of the car so you can see what I'm talking about. Its a black plastic piece, the left side has it and the right side doesn't.

Left (missing): https://i.imgur.com/DiJSfR2.jpg

Right (for reference): https://i.imgur.com/oc656t4.jpg

Is this piece just to protect that bottom nut on the shock from dirt? Does anyone know what this piece is called and where I could get a new one (preferably online since my closest Mini dealer is 50min away)?

 

They are prone to fall off. I wouldn't bother replacing it.

 

Do they have any significant benefit? I live in NY where theres tons of salt in the winter so if it'll help protect I wouldn't mind eventually getting another one.

 

Lots of times, those pieces are meant to keep metal parts from colliding in a rear-impact crash and causing sparks, which could cause a fire if the fuel tank is ruptured in the crash. I work on police cars for a living, and the Crown Victoria Police Interceptor models have similar pieces on some rear suspension components for that exact reason.