Overview:

useCallback returns a normal JavaScript function, regarding how to use it. It is the same function as the one it gets as first parameter regarding what it does. Though the returned function is memoized. Which means when your component re-renders, React will not recreate that function (which is the behaviour for a normal function inside a component).

React will recreate a new version of that function if one of the variables inside the array (the second parameter of useCallback) changes. It's worth it if you pass it down to a component that is memoized with React.memo (something to do if that component does a lot of calculation or really heavy).

A memoized component will re-render only if its state or props changes. And because normally a new version of that passed function is created when the parent re-renders, the child component gets a new reference of that function, so it re-renders. That is why sometimes useCallback is used to memoize the passed function.

Your use case:

In your case, using useCallback for onUploadPress is waste of performance, beacause pickPhotoFromLibrary, an unmemoized function is in the dependency array. It is a waste of performance also because TouchableOpacity is not memoized with React.memo.

Important note:

Memoizing is not free. Bad memoizing is worse than no memoizing at all. Here is the most complete and shortest ressource I have ever seen for understanding in depth, the React render process to know when to memoize and when not to: React Render Tutorial.

In simple words, useCallback is used to save the function reference somewhere outside the component render so we could use the same reference again. That reference will be changed whenever one of the variables in the dependencies array changes. As you know React try to minimize the re-rendering process by watching some variables' value changes, then it decides to re-render on not depending on the old-value and new-value of those variables. So, the basic usage of useCallback is to hold old-value and the new-value equally.

I will try to demonstrate it more by giving some examples in situations we must use useCalback in.

• Example 1: When the function is one of the dependencies array of the useEffect.

function Component(){
  const [state, setState] = useState()
  
  // Should use `useCallback`
  function handleChange(input){
    setState(...)
  }

  useEffect(()=>{
    handleChange(...)
  },[handleChange])

  return ...
}

• Example 2: When the function is being passed to one of the children components. Especially when it is being called on their useEffect hook, it leads to an infinite loop.

function Parent(){
  const [state, setState] = useState()
  
  function handleChange(input){
    setState(...)
  }

  return <Child onChange={handleChange} />
}

function Child({onChange}){
  const [state, setState] = useState()
  
  useEffect(()=>{
    onChange(...)
  },[onChange])

  return "Child"
}

• Example 3: When you use React Context that holds a state and returns only the state setters functions, you need the consumer of that context to not rerender every time the state update as it may harm the performance.

const Context = React.createContext();

function ContextProvider({children}){
  const [state, setState] = useState([]);
  
  // Should use `useCallback`
  const addToState = (input) => {
    setState(prev => [...prev, input]);
  }

  // Should use `useCallback`
  const removeFromState = (input) => {
    setState(prev => prev.filter(elem => elem.id !== input.id));
  }

  // Should use `useCallback` with empty []
  const getState = () => {
    return state;
  }

  const contextValue= React.useMemo(
    () => ({ addToState , removeFromState , getState}),
    [addToState , removeFromState , getState]
  );

  // if we used `useCallback`, our contextValue will never change and all the subscribers will not re-render
  <Context.Provider value={contextValue}>
    {children}
  </Context.Provider>
}

Example 4: If you are subscribed to the observer, timer, document events, and need to unsubscribe when the component unmount or for any other reason. SO we need to access the same reference to unsubscribe from it.

function Component(){

  // should use `useCallback`
  const handler = () => {...}
  
  useEffect(() => {
    element.addEventListener(eventType, handler)
    return () => element.removeEventListener(eventType, handler)
  }, [eventType, element])


  return ...
}

That's it, there are multiple situations you can use it too, but I hope these examples demonstrated the main idea behind useCallback. And always remember you don't need to use it if the cost of the re-render is negligible.