why we use dependcies in react useEffect code example

Example 1: useeffect react

useEffect(() => {
	//  code goes here
    return () => {
      // cleanup code codes here
    };
  },[]);

Example 2: useEffectOnce

import {useEffectOnce} from 'react-use';

const Demo = () => {
  useEffectOnce(() => {
    console.log('Running effect once on mount')
    
    return () => {
      console.log('Running clean-up of effect on unmount')
    }
  });

  return null;
};