It's not conjecture, there have actually been studies about how cats use boxes to reduce stress. One study looked at cats who had just arrived at an animal shelter. The researchers used the Kessler and Turner Cat-Stress-Score to observe the animals. That includes noting activity levels and their ear and tail positions.
Cats who had boxes to hide in saw their anxiety levels reduced in around three days after arrival, while cats without an enclosed space took around two weeks to reach those same levels of reduced anxiety.
Outside of anxiety, it's believed that cats also like boxes because houses are generally a little colder than cats would prefer. Curling up into a box helps them stay warm. That's the same reason your cat probably likes taking naps in a beam of sunlight or lying down near the radiator.
So, if you get a new cat, get them a box to hang out in. It won't stop them from being so lazy, but they'll probably feel a little better about life.
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