saveFieldEpic-v2.js

import { Observable } from 'rxjs/Observable';
import 'rxjs/add/observable/of';
import 'rxjs/add/operator/catch';
import 'rxjs/add/operator/debounceTime';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/mergeMap';
import 'rxjs/add/operator/startWith';

import {
    isSaving, savingSuccess, savingError,
} from '../actions/autosave-actions.js';


const saveField = (action$, null, {ajax}) =>
    action$.ofType('SAVE_FIELD')
        .debounceTime(500)
        .mergeMap(({payload}) =>
            ajax({
                method: 'PATCH',
                url: payload.url,
                body: JSON.stringify(payload),
            })
            .map(resp => savingSuccess(resp))
            .catch(error => Observable.of(savingError(error)))
            .startWith(isSaving())
        );

export default saveField;

Can you explain why use startWith here?