Align the bezel's hour markers to correspond to the time difference that's relevant to you, and
presto: you can read the local time by where the hands are pointing on the dial, and the "away" time according to where the hands are pointing on the bezel (or vice versa, if you choose). For example: NYC is 5 hours behind London. If you travel to London and want to know what time it is back home, you would rotate the bezel 5 clicks to the right until "7" appeared above the watch's 12 o'clock hour marker. Now, the hours in NYC correspond to the hours in local London time. (You simply have to account for AM and PM.)
Even among sport watches with rotating bezels, those with 12-hour markings aren't as common as timepieces with count-up (dive style) bezels — which is unfortunate, because they offer a simpler, more affordable alternative to a GMT. They're not only more useful than the typical three-hander, but the prominent and technical-looking bezel gives a watch that sporty look that's so popular. It's well worth seeking out a watch with this feature, and thankfully, we've been seeing
more and more of them hitting the market lately.