If you have a large amount of memory, are doing decent amounts of IO, and linux OOMs, the system becomes unresponsive for many minutes before killing any process. At which point, ssh sessions timeout endlessly. A serial console stands a chance.

There is then also any case where you're debugging AMI builds and need to fix grub, or the init system without waiting 20 minutes for a new AMI build each time.

Also, the existing console log feature in AWS is insultingly not real time. It doesn't typically update at all unless you're within minutes of boot or trigger a reboot and it only buffers something like 4kb so a reboot can easily fully replace the logs. This really sucks when you're trying to get the debug console output, so this feature finally solves that.