Here's why:

* Tuber: Potatoes are the underground stems (modified stems) of a plant.

* Crop: They are cultivated for their edible parts, which in this case are the tubers.

This makes them distinct from other plant parts used as crops, like:

* Roots: Carrots, beets, turnips, etc.

* Fruits: Apples, oranges, tomatoes, etc.

* Stems: Celery, asparagus, etc.

* Leaves: Spinach, lettuce, kale, etc.

* Seeds: Beans, peas, corn, etc.