What is the purpose of the rubber part on the bottom of your truck

Those are tires. They help to provide maximum traction in all sorts of brutal weather conditions on an usually petroleum based surfaced , sometimes concrete , called a highway.

It , along with your brake system , are probably the two most important features on your vehicle.

 

They are curb feelers. Just look at all the trucks the have them or used to have them. Most are missing or damaged. Thats from not paying attention to where your driving.

 

They reduce drag by keeping turbulence under the truck lower.
As others have noted, they can easily be damaged....and cheaply replaced.
Just another thing to save fuel on trucks, putting less money into OPEC's pockets.

They really need to design a factory removable plastic belly pan like euro cars have had for decades. Then air won't hit everything that hangs on the underside.

 

They're to help with aerodynamics but wonder how much they hurt the aerodynamics when 90% of the ones I see on company trucks are half hanging off and flapping in the breeze.

 

It's aerodynamic trim similar to how the trailer skirts work.

 

I guess it's so they can save $1 a year in fuel by paying $100 for the garb... you get the point.