There's actually no common sense to it. There is absolutely no good reason for washing legs and feet unless they are obviously dirty or smelly. Soap has only been around for a fraction of our existence, and we did fine without it for hundreds of thousands of years.

Washing your hands helps prevent the spread of infection because we touch other people or stuff they are likely to touch, with our hands. Washing your private parts may also help prevent some disease spread, and washing your butt and armpits (and feet if necessary) to remove odour is a current social expectation. Otherwise, there's no need to directly wash any body part unless they are obviously soiled.

On a related matter, as a society we have actually addicted ourselves to detergent shampoo. The more you shampoo and deplete oil from your scalp, the faster the scalp pumps out more oil and makes your hair look greasy. That's why most people need to wash their hair every day - they're in a vicious cycle, where they need to wash their hair frequently to remove grease, but the frequent washing causes more grease. I stopped washing my hair several years ago, and kept it buzzed for the first few months until my body adjusted, and now I only rinse it with water, and it never gets greasy.

Not only that, but by midwinter, my hands used to be raw and cracked and bleeding from the dry air, and since I stopped using shampoo, my hands are much better in the winter, even though I wash them several times a day. Also I no longer have as much oiliness aroung my nose, or dryness of the forehead. So-called "combination skin" is a by-product of shampoo, but the same companies that cause it are happy to sell you additional product to fix it.