When you create a new event you cannot specify a timezone for it.
So for example you're in Europe but you want to create an event at 10am in US Pacific time, it'd be very handy to be able to create the event at 10am and set the specific timezone to the event.

That would be very useful for recurring events as daylight saving happens on different dates in the different timezones. So setting the "main" timezone for an event would be useful, as the event won't ever change time on that timezone, and the rest would adapt as needed.