Why use Next.js API route with an external API?

Question

I am new in Next.js. I want to know what is the use of export default function handler because we can directly call the API using fetch.

In my HTML code I put below code. When I click on submit button sendformData() function will be called.

<input type="button" value="Submit" onClick={() => this.sendformData()} ></input>

sendformData = async () => {
    const res = await fetch("/api/comments/getTwitFrmUrl?twitUrl=" + this.state.twitUrl, {
      headers: {
        "Content-Type": "application/json",
      },
      method: "GET",
    });

    const result = await res.json();
    this.setState({ data: result.data });
  };

When sendformData function is called, it calls /api/comments/ file and calls the function.

Here is the /api/comments/[id].js file code.

export default async function handler(req, res) {
  if (req.query.id == 'getTwitFrmUrl') {
    const resData = await fetch(
      "https://dev.. .com/api/getTwitFrmUrl?twitId=" + req.query.twitUrl
    ).then((response) => response.text()).then(result => JSON.parse(result).data);

    res.status(200).json({ data: resData });
  }
  else if (req.query.id == 'getformdata') {
    console.log('getformdata api');
    res.status(200).json({ user: 'getuserData' });
  }
}

When I put the below code in the sendformData same response will be retrieved. So why we need to call export default function handler function?

 sendformData = async () => {
    const res = await fetch(
          "https://dev.. .com/api/getTwitFrmUrl?twitId=" + req.query.twitUrl
        ).then((response) => response.text()).then(result => JSON.parse(result).data);

    const result = await res.json();
    this.setState({ data: result.data });
  };