why reducers should be pure

pureExample.js

//initial state
const data = {
    id : null,
    password : null
};

function reducer(state = data, action){
    switch(action.type){
        case ID_MODIFIED:
            return {...state, id : action.payload };
        case PW_MODIFIED:
            return {...state, password : action.payload };
        default ...
    }
}