Explanation: The Romans, quite radically for their time, had a habit of extending citizenship to their subjects on equal (or more equal - some towns received 'lesser' Latin right citizenship initially) terms. This is something the Romans themselves recognized as exceptional - the Emperor Claudius muses, in a recorded speech to the Senate, that the downfall of Athens and Sparta was that they never brought the conquered 'into the fold', while Rome was FOUNDED as a city of exiles and criminals.
The process for extending citizenship was informal, initially - the Senate and People would decide whenever they damn well felt like it to extend citizenship - typically either to local elites, or the free population on an entire town/region. The Emperor Claudius gave a nice, formalized way for everyone who wanted it to get citizenship - service in the auxiliary forces of Rome. For a quick, quick term of ~25 years, you, your wife, and your kids (or any kids you'd have in the future, for that matter), would all gain Roman citizenship at the end of your term. And you'd be paid and fed in the meantime! Less pay than the Legions, and no 10 years' extra pay at the end, but hell, a paycheck's a paycheck!
The Romans regarded this, thus, as akin to their idea of the novus homo, a man who, by his own merits, rises to a higher station, and is thus eminently worthy of respect. Seems like a good deal! Just be careful not to get stationed in the east. Or the north. Or the south. Or Britain.