The subtly of a red tuna meat soaked in shoyu gives it an added dimension. This is a particular style is edomae (edo styled) when you see it more heavily browned on the exterior. My guess is these cuts were considered less flavourful and so chefs discovered that by adding shoyu by soaking you can achieve another taste. You will see from shop to shop the various methods of marinating red tuna meat. This is one style. Here is another: https://mesubim.com/2015/03/27/akami-zuke/