Discussions Intakes and turbines on spaceships

What's the purpose of them? A large number of spaceships ranging in size from single seat interceptors to the large Naboo vessels similar in size the Millennium Falcon possess them. Is there an atmospheric engine mode that uses the air in combustion to save on fuel consumption? Many of these vessels spend only small amounts of time in the atmosphere, so I thought it kind of a waste to design such a feature as a major component of an engine.