What is the difference between dmesg and journalctl

I know that dmesg and journalctl record commands logs invoked by my operating-system. From the manpages: > dmesg is used to examine or control the kernel ring buffer. > > The default action is to display all messages from the kernel ring buffer. > journalctl may be used to query the contents of the systemd journal as written by systemd-journald.service > > [...] > > -k, --dmesg Show only kernel messages. **Why do 2 recorders exist**, what types of messages should I expect to see within each of them, and what are the differences in their life cycles?