Unless you are starving, you're probably not going to walk down to the beach and start munching on seaweed. A vast number of plants grow in the world's oceans, but not all of them are safe for consumption, and the ones we eat are usually processed - often by drying.
It's probably through a process of elimination that humans discovered which seaweeds are edible. Once we figured it out, though, seaweed became an important source of nutrition, containing essential minerals, vitamins and iodine.
Many of us are familiar with the dried, green-black seaweed found on the shelves of supermarkets and 7-Elevens, but it comes in many other forms: it's used to thicken liquid, in diet tablets (because it's low in calories) and as agar agar, a vegetarian gelatine. Japanese cuisine features several types of seaweed (and there are many Korean equivalents), such as nori (thin, dried, paper-fine pressed sheets that are often salted and seasoned); kombu (sturdier pieces, often used to make Japanese soup stocks); wakame (thin and slippery, it's used in soups and salads); and hijiki (soaked and simmered with other ingredients, it's eaten as a side dish, a salad or in soups).
When I was a child, I would take a sheet of nori and waft it over the flame of a gas burner to very lightly toast it (it catches fire if held too close to the flame), then eat it as a nutritious snack, sometimes with rice. My mother liked to make a light soup by toasting nori sheets, tearing them into smaller pieces then adding them to chicken broth that she would simmer with cubed bean curd, sliced daikon (white radish) and a little sliced meat.
If you're wrapping onigiri (rice balls) in nori, do so at the last minute, or the seaweed will become soggy.