A gate with a ratio control becomes an expander.
Expanders are usually used when you want to make the softer parts even softer - but not cut them out completely. SureShot explained it pretty well. Undesired background noise might be something you would experiment using an expander on, to see if the result is better than a pure gate.
As to your other question, yes, sometimes other tracks can cover up noise or mask decays, but sometimes you may be recording music that uses more space or is otherwise more transparent. There are more things in life (and music) than kick-ass balls-to-the-wall rock and roll.