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The pillow is probably activating a reflex to press the legs together.

In other news, it's the Stabile Seitenlage [1] from first aid! https://en.wikipedia.org/wiki/Recovery_position

At least for me it's comfort - I don't like my boney knees knocking into each other.

Probably to help align the hips and lumbar spine, otherwise the top leg would twist your pelvis and spine with the downward pressure.

And it works wonders. Nothing repairs my tired back faster than a few dedicated nights of sleeping like that.

I have heard before another reason is that it can help keep you from rotating into a different position. That's why I do it. Without the pillow I have 0% chance of staying on my side.

Hip alignment.