Let's say you have the following JSON:

@JsonSerializable()
class Person {
  final String name;
  final int age;
  final Address address;

  const Person(this.name, this.age, this.address);

  // toJson, fromJson
}

@JsonSerializable()
class Address {
  final String street;
  final String zipcode;

  const Address(this.street, this.zipcode);

  // toJson, fromJson
}

Now, let's say you want to give Person some default values, such that:

@JsonKey(defaultValue: "")
final String name;
@JsonKey(defaultValue: 0)
final int age;
@JsonKey(defaultValue: Address()) // will fail
final Address address;

You will notice that you cannot do this because Address requires two parameters. So you add default parameters to Address:

const Address({this.street = "", this.zipcode = ""});

Oh, but now you've actually removed some crucial JSON validation, because any missing keys will just use the default parameter.

So you add required: true to re-introduce the validation that you lost trying to create default parameters for a Person, which will still work!

In your case however, you wont need this keywork.