What are cowl flaps?

What are cowl flaps for, and how are they used? The name implies that
they are some sort of aerodynamic control surface, but they don't
appear to have any aerodynamic effects. In simulation, they are doors
on the bottom of the engine nacelles that open for some unclear
purpose (I see what looks like exhaust pipes or something inside).
What do they do, and why do I have a control for opening and closing
them? This is on a Baron 58, the aircraft I usually fly in
simulation.

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