Earlier quoted context omitted.
The point of the date it to tell people when the material was created. In some jurisdictions / situations, copyright expires N years after it was created, so that date tells people when copyright has expired. Unfortunately, in many cases now, copyright is based on the year the author died, rather than the year the work was created, which means it is much harder to work out if something has entered the public domain.…
doesn't moving the creation date forward in time make you vulnerable? if you create some content in 2018 and I rip it off in 2019 and then you update your copyright to 2020, what's to stop me from saying I came up with it first?
That's partly why you have a span like "Copyright 2018-2020" - you are saying some of this material is older, some isn't. You need to keep copies of the older version with the older copyright to be clear which parts were invented when.