Why is marlin sold in sauce, not plain?

I am currently staying near Mito in Ibaraki, and don't know if this is only a local phenomenon:

The local grocery stores sell marlin (hajiki) in some sort of red sauce/marinade. Why? What is the composition of the sauce/marinade? They also sell another kind of fish in a white sauce/marinade. I don't know what kind of fish this is, or what the ingredients of the white sauce are. Perhaps someone can tell me?

All other fish is sold plain. What is special about these two?

Thanks!

One supermarket close to where I live sell "kajiki" fillets marinated in... soy sauce and "mirin" flavor. Why? It makes a quick main dish, you just grill the fillets in a frying pan, it's already flavored, so it's ready in a few minutes. If you say reddish sauce, that's one flavor I can think of. I don't know what the white sauce is... you will need to ask the shop.

The above is sold in "processed fish products" section, and they have another fresh fish section where they sell plain fillets too.

I guess that fish is often comes to the fish shop in frozen slabs, not much flavor on its own (though I buy the plain fillets, salt & pepper, grill, and add some other flavoring myself too, though), so they just want to sell them half-prepared for customers' convenience.