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.

Changing the date without changing the code significantly is arguably fraudulent.