It’s dreadfully inconvenient to compare logs on two systems and find out they don’t come close to agreeing when “now” is.
When there are things talking to each other, and time is exchanged, it’s often critical for deciding which time came before the other. That needs syncnronization.
It’s really awkward when using a NAS appliance that has times on files that don’t agree with clients. Things that compare file times (like make) get very confused. Editors refuse to save files, because they think the old version is newer than the one you just changed.
It’s usually done with NTP, the network time protocol, but there are alternatives. Among them are the use of GPS receivers and rack-mount atomic clocks. But they typically act as an NTP server inside a server complex.