A calendar appointment isn't identified by a UTC instant; it's identified by a predicate.
(1) event scheduled for 7:00pm local time in a certain place on a certain date, that needs to be at that local time regardless of whether the government changes to time zone of that locality, or changes DST rules.
(2) event at 7:00pm Central Time ona certain date, that needs to occur at that time in the regardless of DST date changes.
(3) event at 7:00pm CST, which has a fixed offset from UTC.
But it's only a small step farther to allow scheduling an appointment relative to an arbitrary time zone, instead of the one you happen to currently be in. Which naturally includes UTC, for situations where astronomical time is what you really care about.
Of course it can't happen by magic, and you rely on the authors of the timezone libraries (a thankless task in itself), but if you remove a dimension of your data, nobody can ever help you.