Aesthetics? Also, when it comes to mechanical watches, the argument of “why would anyone buy a more expensive [X] than a less expensive [Y]” seems to be a dangerous line of thought...
Admitting first that all mechanical watch functions are basically “useless,” GMT is the least useless for me (and I suspect, many others): I travel through different time zones a lot, and when I’m not traveling I’m typically tracking the time zones of those I need to be in regular contact with. In fact, I live in central U.S., leave my “home” time set to Sydney, Australia, and when I travel bump my “local” time to my destination. In fact, I’m looking to better understand using a GMT bezel for 3-zone tracking... (home, local, and Sydney)