Metal, water and air don’t play well together. Why is that? It is because that combination can produce rust. Once rust starts, it will slowly eat away at the metal until it is structurally unsound. You can see why that would be a problem to a homeowner. There are many places where rust can cause serious problems. For example, if you have cement with rebar inside it, and the rebar gets exposed and starts to rust, then you will have the problem of the rebar expanding, causing the cement to crack more and exposing more rebar.
If your roof were to rust, you could soon have leaks, which could lead to water damage inside your home.
You can see why it is important to care for issues with rust as soon as possible. You will also need to clean it up completely, since leaving just a little bit of rust behind will let the process continue and become a problem once again.