You've seen the effects of rust. It causes a loss of untold millions of dollars each year.
Rust is in general, the oxidation of a metal surface. It's a chemical reaction that happens when oxygen in the air or water reacts with iron in metal.
The problem with iron (as well as other metals) is that the oxide formed by the oxidation does not adhere well to the surface of the metal and flakes off. This leaves a pitted surface on the metal. Eventually, if nothing is done, the pitted surface weakens the metal structure.
Removing the rust and then sealing the metal to prevent the further reaction of oxygen with the metal, is one method to halt the process and save the metal structure.