So a DNS zone file contains an SOA record specifying the authoritative name server, along with some other information such as a "serial number".
I'm trying to understand the purpose of the serial number. Googling around for answers, I found the following explanation:
Serial numbers in DNS zone files provide a way for the server to verify that the contents of a particular zone file are up-to-date. If the serial number in a zone file hasn't changed since that zone was last loaded, named figures that it can ignore the file. This means that sysadmins have to remember to update the serial number every time they make a change to a zone file -- otherwise, their changes won't be picked up and published.
I don't understand this explanation, and the fact that this requires sysadmins to remember to manually update the serial number seems insane.
So, what's the purpose of the serial number? It can't be for other DNS servers to verify if a particular record is out-of-date, because that's what the TTL is for.
So, why is it important to have a serial-number? Why not just have the DNS server re-read the zone file when a change is made?