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An accurate clock is essential for the navigation.
For every 15 degree that one travels eastward, the
local time moves one hour ahead. Similarly, traveling
West, the local time moves back one hour for every 15
degree of longitude. Therefore, if we know the local
times at two points on Earth, we can use the difference
between them to calculate how far apart those places
are in longitude, east or west.
In the 17th century, sailors and navigators can find
out the local time by observing the Sun, but finding
out the longitude require the time of a reference point,
e.g. Greenwich.
Although accurate pendulum clocks existed in that time,
these clocks could not keep accurate time at sea because
of the motions of a ship and changes in humidity and
temperature.
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