This time zone is used for webinar and registration start dates. Our events run in user's local timezone. That means if you set up an event to run at 3pm everyday, and your attendee A lives in Arizona, US and attendee B lives in Madrid, Spain, both will see the webinar at 3pm at their own timezones.
However, that represents a problem for the "Live" or "Hybrid" events, right? That is why we have this "Webinar Timezone" settings. If you are running your webinar live/doing a hybrid event, you can set the timezone here and that's when your live/hybrid webinar will start.