Why use timezone for start & end datetime in events

See more:

Javascript

Google

calendar

I know it is ideal practice to store event start & end datetime values in the database table by converting from local time to UTC time. And then while displaying it on the calendar convert it back to the local time or user prefered timezone.

But I can't understand why is Google storing timezone name for start & end datetime in the below code

var event = {
  'summary': 'Google I/O 2015',
  'location': '800 Howard St., San Francisco, CA 94103',
  'description': 'A chance to hear more about Google\'s developer products.',
  'start': {
    'dateTime': '2015-05-28T09:00:00-07:00',
    'timeZone': 'America/Los_Angeles'
  },
  'end': {
    'dateTime': '2015-05-28T17:00:00-07:00',
    'timeZone': 'America/Los_Angeles'
  },
  'recurrence': [
    'RRULE:FREQ=DAILY;COUNT=2'
  ],
  'attendees': [
    {'email': 'lpage@example.com'},
    {'email': 'sbrin@example.com'}
  ],
  'reminders': {
    'useDefault': false,
    'overrides': [
      {'method': 'email', 'minutes': 24 * 60},
      {'method': 'popup', 'minutes': 10}
    ]
  }
};

var request = gapi.client.calendar.events.insert({
  'calendarId': 'primary',
  'resource': event
});

request.execute(function(event) {
  appendPre('Event created: ' + event.htmlLink);
});

What I have tried:

I am trying to create a simple alarm app with timezone support. I am storing datetime values in the DB table by converting them from local time to UTC