They say it helps relax the astronauts but really it is about superstitions and warding off bad luck.
The NASA Artemis blog states - "NASA crews play cards until the commander loses. It is hoped that by losing, the commander burns off all his or her bad luck, thereby clearing the mission for only good luck."
There are many such rituals in the space community. See https://www.thespacereview.com/article/1137/1
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