Credit: National Air and Space Museum, Smithsonian Institution
Some spacecraft are given a spin, like a gyroscope, to keep them oriented in a certain way. You can also see this same effect at work on Earth. A bicycle is easier to balance while moving because its spinning wheels help stabilize it. The spinning action of a top also stabilizes it, and helps keep it from falling over (at least until drag slows its spinning and gravity pulls it to the ground).
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Flight Dynamics