When using fetch api and set headers I can just use object literals or using a Headers object,

const response = await fetch("https://example.org/post", {
  headers: {
    "Content-Type": "application/json",
    ...
  },
});

// Or

const myHeaders = new Headers();
myHeaders.append("Content-Type", "application/json");
const response = await fetch("https://example.org/post", {
   headers: myHeaders,
});

As MDN document says,

You can pass an object literal here containing header-name:
header-value properties, Alternatively, you can construct a Headers
object, add headers to that object using Headers.append(), then assign
the Headers object to the headers option

I check Headers document but I can’t find the advantages of using it. So why bother ?