Because network-synchronized time clients are now nearly everywhere, the cost and accuracy of onboard computer clocks has declined over the past decade.
At the extreme, Raspberry Pis don't even have an integrated battery-sustained clock, and if the Pi has no network connection, it has no time at all.
> How much would its clock/time deviate, especially compared to a quartz or mechanical watch?
In my experience, quite a lot. To see this for yourself, log two machines onto http://arachnoid.com/NaturalResources/big_clock.html. On one machine, make a local copy of the page and disconnect from the Internet. See how quickly that machine deviates from correct time.
At the extreme, Raspberry Pis don't even have an integrated battery-sustained clock, and if the Pi has no network connection, it has no time at all.
> How much would its clock/time deviate, especially compared to a quartz or mechanical watch?
In my experience, quite a lot. To see this for yourself, log two machines onto http://arachnoid.com/NaturalResources/big_clock.html. On one machine, make a local copy of the page and disconnect from the Internet. See how quickly that machine deviates from correct time.