People who grow their own can usually only grow seeded watermelons. Seedless usually have to come from slip cuttings.
Also, seeded watermelons are usually bigger. If you are serving a crowd, some people buy the larger ones.
I prefer the smaller seedless (well mostly seedless) watermelons because where we live, they are sweeter. Also, I don't have to store a bigass watermelon in my refrigerator if it isn't finished.
I wanted to add watermelon plants can't grow in your belly. It is too acidic, doesn't have access to soil and the right kind of nutrients and most important; there is no sunlight in your body. Plants need sunlight to to more than just sprout. In most people's bodies their upper GI system is simply too acidic to sprout plants.