Redux Reducers Example

reducer.js

function reducer(state = []) {
  return [...state, {
    title: "Test",
    completed: false
  }];
} 

You're returning a NEW array (new reference because data changed)
But most internal objects are references to the ones provided in the state argument (spread operator (...)), and you add a new object to the array.

So you only created a new array, kept reference of all previous data, and created a new object within the array.

The "new array" is created to change the reference, allowing most store subscribers to "react" by simple comparison (if (previous !== new) do something), so you don't have to do deep comparisons, which can be costly.
Redux + Immutable data structures are FAST, because they work via reference instead of deep copying and deep comparisons (which are the costly bits in most UI-centric apps)