In addition to the more concrete reasons, abstractly they're getting a bit of extra usage out of the namespace by segmenting it. A 9ZZZ999-type license plate is not just any license plate — it's specifically an ordinary private vehicle (as opposed to a state-owned vehicle or a trailer) that was registered in California between 1980 and 2026, and both of those facts are durably encoded in the number. Notably, both of these are also very human-readable facts, which for most of the existence of the car bureaucracy was extremely germane. The CA DMV got its digital-records act together in the 1990s (this is from memory, it might have been in the Bush years but it certainly wasn't in the '80s and it was a done deal by the Obama era) but there was a long time before that when "just plug it into the DB" was not an option because the DB was a filing cabinet and the query engine was a human digging through it.