My grandparent’s cats slept in their plant pot when it had nothing in, they only did it often in summer. I wonder why they would want their pelts to be dirty with grit and dirt and weeds.
Same reason they sleep in other locations - warmth and comfort.
Plant pots containing soil warm up in the daytime in summer, the soil and pot retaining that heat (especially if they're terracotta pots). It seems natural that a cat would find a plant pot an attractive place to stay.
Being round allows the feline to curl up comfortably as well.
They don't especially want dirt and grit on their coats, but that's something they can deal with afterwards (most of the time they'll probably shrug the dirt right off).