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The bezel just makes it easier to read, and I guess it gives it the "sporty" GMT look. The earliest GMT-Masters relied on the rotating bezel to display a second time zone as the 24hr hand couldn’t be set independently. Once it could, they retained the rotating bezel anyway, which gave it the ability to track three time zones rather than just two.
Either way works. A bezel is quicker to adjust, but the GS GMTs do what modern Rolexes do which is to have a jumping local hour hand that you can change forward and backwards without stopping the watch or affecting the minutes or the “home” GMT hand. Pretty neat.
Don’t really get the point of having a GMT bezel that doesn’t rotate.