why do you combine reducers in redux code example

Example 1: combine reducers redux

// reducers.js
export default theDefaultReducer = (state = 0, action) => state

export const firstNamedReducer = (state = 1, action) => state

export const secondNamedReducer = (state = 2, action) => state

// rootReducer.js
import { combineReducers, createStore } from 'redux'

import theDefaultReducer, {
  firstNamedReducer,
  secondNamedReducer
} from './reducers'

// Use ES6 object literal shorthand syntax to define the object shape
const rootReducer = combineReducers({
  theDefaultReducer,
  firstNamedReducer,
  secondNamedReducer
})

const store = createStore(rootReducer)
console.log(store.getState())
// {theDefaultReducer : 0, firstNamedReducer : 1, secondNamedReducer : 2}

Example 2: adding multiple reducer to rootreducer using combine reducers

import { combineReducers } from 'redux';
import { registerReducer } from './registerReducer';
import { loginReducer } from './loginReducer';

export default combineReducers({
  registerReducer,
  loginReducer
});