People want to understand when things are open/reasonable without having to do a lookup every time. A conversion has to happen somewhere - either I can say "X is in timezone foo. It's 1300 there so they're awake and I can call them" or "It's 1900 UTC, X is awake from....err....2200 to 1400, so I can call now".

The first is significantly easier as it requires remembering only a single offset and then going with societal conventions.

> "It's 1900 UTC, X is awake from....err....2200 to 1400, so I can call now"

Assuming it's that simple of course. Like, you can do mental math about what UTC "officially means" for someone many miles away, but people coordinate with others even if it means their local schedule is not aligned with that ideal case. Time zones account for this by being wider or narrower than their Platonic 15° of longitude ideal in some places.