The value is not required, that's why it says "OPTIONAL" in capital letters at the link you gave.
WebAuthn has two scenarios. For something like an iPhone or a Windows laptop we can choose to have the device store a heap of credentials mapped to domain names. So your Safari sees this is cat-videos.example and the iPhone knows credentials for cat-videos.example so .get() fetches those credentials without needing IDs and the site doesn't (if it does things this way) need to ask for a username first. This is called "resident key" because the device remembers which keys it has and you can have your site say during registration that you prefer this outcome, or even that you insist upon it by setting it as "required".
The other scenario is older and very clever, it is used for things like a cheap FIDO USB dongle. It stores nothing, the device doesn't know who you are. This means it cannot provide that seamless "no need to enter your username" UX, however it does work well as a second factor and can be used as a sole factor if you want. How can it work if the device doesn't remember credentials? Well the WebAuthn ID is deliberately big enough to "hide" encrypted keys inside it. Instead of remembering your credentials after registration like this Safari feature, a cheap FIDO dongle encrypts the private key it'll need and then gives that to the web site to store as an ID, alongside the public key. When you give the site your username, it fishes out the IDs associated with your user and hands those to get() in that buffer you spotted. The user's web browser shows these IDs to any FIDO dongles and if one of them made that ID it can decrypt it successfully and authenticate you.
So the overall answer to your question is: It depends.
The cheapest simplest WebAuthn setup uses it only as a second factor and would need a username every time.
A very nice UX focused on being as seamless as possible for Apple users wouldn't need a username unless you're using a device you haven't authorised this way before. The video shows such a UX because that's what Apple wants you to do for their users.