You can build system that do not require physical clock synchronization, but using physical clock often lead to simpler code and major performance advantage.
That's why Google built True Time, which provides physical time guarantee of [min_real_timestamp, max_real_timestamp] for each timestamp instant. You can easily know the ordering of 2 events by comparing the bounds of their timestamps as long as the bounds do not overlap. In order to achieve that, Google try to keep the bound as small as possible, using the most accurate clocks they can find: atomic and GPS clocks.
That's why Google built True Time, which provides physical time guarantee of [min_real_timestamp, max_real_timestamp] for each timestamp instant. You can easily know the ordering of 2 events by comparing the bounds of their timestamps as long as the bounds do not overlap. In order to achieve that, Google try to keep the bound as small as possible, using the most accurate clocks they can find: atomic and GPS clocks.