Most vehicles should have a license plates, if bought through legal means. The plate numbers provides you legal referent and legitimatizes that this vehicle is owned by you.
No plates are good, for doing crimes.
Topside should have plates on all their vehicles and if they don't, then most likely thy will end-up in WJF impound lot.
Without getting too deep into specifics: if you've noticed a vehicle lacks plates topside and is getting away with it, there is almost certainly an IC reason at play. It's up to you to figure out what's going on.