It's nice to see a proposal that fits so neatly within existing infrastructure and norms. I just hope they either standardize or drop the extensible type field idea instead of letting it turn into a semi-documented mess other projects can't easily adopt.
A dependency being optional on a technical level doesn't mean that it's optional on a functional level for everyone. `libfido2` mentioned in the pull request is a great example: if your rootdisk is encrypted using a FIDO token, you need that library in the initrd; but otherwise you don't. By making the dependency optional and initrd generators aware of that optional dependency, people that need it will have things just work, while others don't need to install the library.
You know that you use a token to unlock your boot device. You don't necessarily know the specific set of libraries needed to enable the feature, though.
Because the optional dependencies actually do things. By default an initrd-builder would probably copy all deps including optional ones but it now enables you to choose not to do that as well, an avenue that was previously not available.
