The way I've always looked at it is that standard drivers license for private vehicles.
Any job related vehicle you can operate solely with a careers drivers license, without having a standard one at all.
This doesn't make a huge amount of sense from a realism standpoint, granted. Realistically the careers license would most likely be something akin to a CVE or similar.
However, the reason I see them as two separate licenses that can be used independently of each other, is simply for the sake of new players.
The majority of starter jobs (food truck, tow truck, taxi, delivery, cop etc.) all have job vehicles that are more or less hard requirements for doing the job. Without a license new players would be severely handicapped.
The careers drivers license is both significantly cheaper and easier to get than the standard one, catering more to new players.
If a career driver had to have both, it would be a lot more difficult for new players to start out.
They would have to get an even larger amount of money to be able to access basic starter jobs, not to mention that they would have to figure out even more stuff to get started on an already overwhelming gamemode for new players.
If the licenses should be changed to operate where the validity of a careers license was dependent on the status of your standard license, the license system itself would have to be changed to accommodate new players first.
I do agree with
@DystopianRaven though, that we should make the distinction between the two clearer for players.