All fields in GraphQL are nullable by default. Hence it's a good practice to mark arguments required for query execution with GraphQLNonNull or in case of SDL with an exclamation mark – String!. This will allow detecting certain errors on the client at static analysis stage instead of a runtime stage.

In addition, if an argument is defined as required a resolve function on the server might not need to check for its presence. Т.к.

For example, to get a list of articles a consumer has to specify the number of records to return:

type Query {
  articles(limit: Int!): [Article]
}

You can specify the default value for the argument:

type Query {
  articles(limit: Int! = 10): [Article]
}

In this case, even though an argument is required it can be omitted by the consumer while the backend gets its required argument set to 10. A combination of a required argument with a default value guards against cases when a consumer tries tricking the system by providing a null value - query { articles(limit: null) }. Thanks to the required definition the server will throw an error Expected type Int!, found null..

Required arguments in GraphQL work good and you should use them to define your GraphQL schema in a stricter way.