[nisse72]

A commenter to the wikipedia page I posted about the vancouver 9 o'clock gun makes a compelling point: it was all about providing a time signal for ships in port to set their onboard clocks.

[marcinzm]

That makes sense. Ships used time to help get their location back then and presumably even a second deviation could put you off course.

Ships also used Greenwich Mean Time (which this cannon is tied to) and astrological data for local time (which this cannon also used for its time).