H. H. Scullard has one of the best discussions of the "hierarchy of cities" in the Roman Empire. I'll give you just a sample, which discusses the ius Italicum cities and municipia. You can check out the rest in the book if you're interested. It is very interesting. Here's what Scullard writes:
"The status of the cities in the provinces varied greatly, from colonies and municipia to 'Latin' cities and the great bulk of the 'stipendiary' cities. The most privileged were not, as under the Republic, civitates foederatae (though some of these survived), but those that had Roman citizenship, i.e., colonies and municipia. Before the time of Julius Caesar the idea of establishing Roman towns outside Italy was not popular, and Narbo was the only example: Roman citizenship was given to individuals in the provinces but not normally to cities. Caesar broke away from this narrow convention with his numerous overseas colonies for veterans and the poor, and Augustus, though less liberal in his ideas of the wisdom of widespread grants of franchise, followed Caesar's colonizing policy. At the top of the hierarchy of cities stood the colonies: some of them were immunes and probably did not pay tributum capitis, and a few had the privilege of Ius Italicum which excused them from the land tax. Next came the municipia which were existing cities that had been given Roman citizenship, not settlements of immigrant Romans: thus Gades received the title and privileges from Caesar. In practice one chief difference between municipia and colonies was not that the magistracies in the former were less uniform, but that the prestige of the latter was higher; the municipia gradually (but more especially in the second century) began to seek the status and title of colony. They spread through the western provinces, especially in the Mediterranean regions, but this status is not found in the east until much later. . . ." (From the Gracchi to Nero: A History of Rome 133 BC to AD 68, 5th ed. [New York: Rutledge, 1982], 263)This ius Italicum status was the highest status that a town/city outside of Rome could receive in the empire. Citizens in Philippi were really much more than "Philippians." They had their names written in the tribus Voltinia, one of the rural Roman tribes. Even though they had not been born in Rome, they were considered citizens of the glorious city of the Empire. Consider what the people said in Philippi before the city's magistrates, which ultimately led to the imprisonment of Paul and Silas: "These men are throwing our city into confusion, being Jews, and they are proclaiming customs that are not lawful for us to accept or to observe, since we are Romans" (Acts 16:20–21). Imagine what everyone thought when Paul let them know they were actually Romans as well: "'They have beaten us in public without trial, men who are Romans, and have thrown us into prison' . . . They were afraid when they heard that they were Romans" (Acts 16:37, 38). Joseph H. Kellerman writes, "[T]he abbreviation VOL (Voltinia) is seen on half of all first- and second-century inscriptions found in the city [i.e. Philippi]" ("The Humiliation of Christ in the Social World of Roman Philippi, Part 1," BibSac 160 : 329–330).
Leading into the first century, these cities were often connected to veteran settlements initiated and purchased by Augustus. The status would also be passed on to cities as a reward for some manifest devotion to the Roman Empire. In this sense, cities could work to earn the favor of an emperor and win this privileged status––in theory anyways––though it was only granted on rare occasion. Citizens could own, rent, and sell/trade land, and, more importantly, they did not have to pay [certain?] taxes. This law of exemption from taxes for Romans began in 167 B.C. The extension of the status to provinces had as its chief benefit these tax exemptions. The land tax was called the tributum soli. There was another tax called the tributum capitis or poll tax. It gets a little fuzzy as to what exemptions applied to ius Italicum cities. There probably needs to be another study on this issue that peels back the go-to talking points, which get regurgitated in nearly every commentary on Philippians. For example in some literature you'll read they were exempt from all taxes, in other literature that they were exempt from certain taxes. It's just fuzzy. In any case, whoever owned the gold mines in Philippi was probably really, really, really happy to have this right of Italy status. Also, there was in these cities an attempt to replicate all things Roman. The empire wanted to export Roman life and thought to every inch of the empire, sure, but there was something different about these cities. The idea was for someone to experience Rome from afar via a mini-Rome.
Citizens had a huge sense of pride too. Consider how quickly the people in Philippi called attention to their Roman status in Acts 16 (see number one above). Status was extremely important on both the macro and micro levels. For example, their status in the empire (macro) set them above and apart from other cities that had not received ius Italicum status. Within the city, because of the prestige associated with this status, there was a constant struggle to "climb the ladder." Think about it this way. More people are fighting their way to the top in New York, New York than in the small town of Randolph, New York. It doesn't mean there isn't always a struggle for honor and authority; of course, there is. But the prestige associated with certain cities makes that struggle and desire for positions of influence all the more desirable. Paul recognized this. In his letter opening, Paul makes sure to address both the "overseers and deacons" (Phil. 1:1), the only letter opening which mentions these groups. This could be connected to the the veteran presence in Philippi, i.e., there was a "chain of command" that Paul understood he needed to observe if he wanted to connect with them. What's most interesting though is Paul places a title of dishonor in conjunction with his and Timothy's names––"slaves" (1:1). He will of course return to this matter in other passages, none more important than Phil. 2:6–11. And the presence of his name aside Timothy's goes primes his audience for heart of the matter he plans to address in Philippians 4 with the two women that are not living the way a heavenly citizen ought to live. It's also quite interesting that Paul does not refer to his Roman citizenship in his list of credentials in Phil. 3:5–6. Already he is modeling for his Philippian audience what it means to view that status and that citizenship with all of its benefits as paling in comparison to the greatness of being a citizen of Jesus' kingdom.
You can't really get a full picture for what Paul is saying to the Philippians unless you understand this special status experienced by the Philippians in the first century. It's just one of the examples of why we need to do thorough historical analyses of New Testament texts.