There also are ingredients in the coating that help prevent rusting while the water is evaporating. You may not find many applications for these products in residential and commercial applications, but some good ones are those areas where it's difficult to get at the rust to clean it off. For example, it's difficult to get into the seam to clean and prime where two pieces of metal butt against each other. Instead, you can apply a converter, which will run into the seam and eliminate the rust. If you have a large area that's easy to get to, you're better off to clean it and use a standard primer.
Rust converters are substitutes for primers, but they come in very handy in some situations.
No matter what amount of rust you're facing, getting a surface that's as sound and clean as possible will help ensure that the coating will perform. It's going to take more work on some surfaces, but that's the sort of thing that professionals are paid for. Rust isn't ever going to go away, but ironically, that's good for business. If it didn't exist, you'd have a lot less to paint.