Why does the React useEffect Hook need a useState call instead of a variable? – Reactjs

For example, from the documentation (https://reactjs.org/docs/hooks-effect.html) –

import React, { useState, useEffect } from 'react';

function Example() {
  const [count, setCount] = useState(0);

  // Similar to componentDidMount and componentDidUpdate:
  useEffect(() => {
    // Update the document title using the browser API
    document.title = `You clicked ${count} times`;
  });

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

However this would not work:

import React, { useEffect } from 'react';

function Example() {
  var count = 0;

  // Similar to componentDidMount and componentDidUpdate:
  useEffect(() => {
    // Update the document title using the browser API
    document.title = `You clicked ${count} times`;
  });

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => count = (count + 1)}>
        Click me
      </button>
    </div>
  );
}

What requires us to use the useState call? What are the advantages?