why reducers should be pure

noPureExample.js

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

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