First it’s important to note that the species of turtle used in suppon cooking is on the list of threated species in the vulnerable category (third highest category of five that reflect the risk of exctinction).
It is considered a delicacy in Japan, and is high in minerals. Historically it has also been said that it boosts energy during fatigue and sickness and it is supposedly popular among woman for its high collagen content. Suppon itself is served in a variety of ways including in nabe, fried, and in zosui (a mild thin rice soup). It also sometimes appears in school lunches.