为什么我们把扩散算子用于redux

我看到了以下代码:

export default function productReducer(state = initialState, action) {
    switch(action.type) {
        case FETCH_PRODUCTS_BEGIN:
            return {
                ...state,
                loading: true,
                error: null
            };

        case FETCH_PRODUCTS_SUCCESS:
            return {
                ...state,
                loading: false,
                items: action.payload.products
            };

        case FETCH_PRODUCTS_FAILURE:
            return {
                ...state,
                loading: false,
                error: action.payload.error,
                items: []
            };

        default:
            return state;
    }
}

但不明白为什么我们每次都要用……陈述每一个案例。

我们不能只写:

return {
                    loading: false,
                    error: action.payload.error,
                    items: action.payload.products
                };

有人能解释吗?