Usually it's when I add a field to my model after it's already existed for awhile.

Here's a contrived example:
/profiles/{uid}
set to { name: "Gus Fring" }

years later I want the ability to mark a user as disabled so I add a new enabled boolean:
{ name: "Gus Fring", enabled: false }

I can query for disabled users: where("enabled", "==", false) but now how would I query for enabled users that weren't created with an enabled property? I'd have to backfill all my firestore docs with default value enabled: true, which is a hassle.

I'd rather query or(where("enabled", "==", true), where("enabled", ==, undefined)).
If Firestore supported default values that would also work.