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Basically, at times where you are mapping state to a component. If the state being passed is a new object or array with the same data, then the component will render again. So, reselect will prevent a new object, array, etc. from being created when the data does not change.

During times where the selector is in a reuseable component:

// The reuseable component file
// ...
function ReuseableComponent({ someState }) {
  return <somehtml>{someState}</somehtml>;
// A component that uses this reuseable component
// ...
function componentThatUsesReusableComponent() {
  return (
      <ReuseableComponent someState={'this is some state'} />
        someState={'this is some other state with the same state name'}

And the createSelector function is used to compute someState, it will cause a re-render each time. The reason for this is that the createSelector function is the same function in both instances of the reuseable component. So, it is continually altering the state.

The way to prevent this from happening looks like this:

const makeGetVisibleTodos = () => {
  return createSelector(
    [getVisibilityFilter, getTodos],
    (visibilityFilter, todos) => {
      switch (visibilityFilter) {
        case 'SHOW_COMPLETED':
          return todos.filter(todo => todo.completed);
        case 'SHOW_ACTIVE':
          return todos.filter(todo => !todo.completed);
          return todos;

So that a new instance of createSelector is instantiated for each instance of the reuseable component.