Yeah but in your example, you wouldn’t need to look anything up either. You’re presumably very familiar with the offset of your time to their time? You’d also become familiar with their “universal time” versus your time. You’d just know what hours they’d be awake and asleep because you will have done the translation a few times.
In addition, I - personally - would find it easier to memorize times in a single system: e.g. remembering that people in China are awake from 9pm to 8am is easier for me to remember. I typically already do this in my own head. I’ll convert times to my own local time and then memorize that. Do other people not do that? I find it much easier to look at my own clock and know if I can reach out to someone internationally.