Other than that, additional domains also give leverage for diversification if you decide to make them publicly visitable one day.
Genuine question: Is there any tangible/technical benefit to this, or is it just for the sake of keeping them organised?
I can’t really think of anything else that would really benefit from having its own domain though, especially if your DNS infrastructure supports nested zones.
The reason is pragmatic: if a company provides such kind of services then it may fall under a (false) scrutiny one day after somebody uses one of its services for, say, planting a stealth DDoS attack. The whole domain may be yanked or denylisted, and the last domain you want to see under the fire is your .com domain.
The same goes to .org: you do not want to see outsiders messing with your organizational endpoints and internal operations, so you better move them to a separate domain for the sake of a) clarity and b) risk hedging.
There are two drawbacks though: 1) higher domain costs 2) if you are not careful enough in managing expectations, people may be confused about which one to use and when.