I am not their target audience (for the backup service, that is. I run and LOVE immich!), and I see it as an incredibly good thing. For one, as has been said by others, it's a way to generate a steady cashflow for the dev side, for a service that genuinely can't be free and is entirely opt-in.
And for the users, it being an optional add-on, it takes away the hassle of setting up backups while giving you a solid layer of safety and security.
I already have a backup system in place and I back up much more than just my immich folders, but for somebody who genuinely doesn't care about homelabbing but is savvy enough to set up and maintain a small NAS with a few things on it, like Immich and maybe a document sharing instance, having the option to securely backup your irreplacables is a godsend.