what does useeffect do react code example

Example 1: useeffect

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

  useEffect(() => {
    longResolve().then(() => {
      alert(count);
    });
  }, []);

  return (
    <div>
      <button
        onClick={() => {
          setCount(count + 1);
        }}
      >
        Count: {count}
      </button>
    </div>
  );
}

Example 2: react useeffect

import React, { useEffect } from 'react';
	
	const [data,setData]=useState()
    useEffect(() => {
        fetch(`put your url/api request in here`)
        .then(res=>res.json())
        .then(json => setData(json))
    },[])