There is a history for each place (some are lawful, others controversial and some outright imperialism).

For instance the Spanish Canaries are a peculiar case. They were largely uninhibited for most antiquity (6th BCE Century-5th Century), though the islands were found with ruins during the period, suggesting that a sort of civilization might have passed there before antiquity. In any case we're unsure if the islands got repopulated during the middle ages or if there always lived a few indigenous people whose the antiquity civilizations never found when they traveled to the island, likely the last (though this show us how few they were).

In either case Europeans began to explore the island around the beginning of the 14th century and they found there indigenous peoples that they called Guanches which lived with stone age's technology. They were extremely few in numbers as well, just what would you expected from hunter-gatherers tribes. The Castellan Crown (though mainly its nobility independently) began to conquest and colonize the island around 1402 and while the natives resisted the frankly speaking minimal armies sent abroad, by 1495 the islands were fully conquered and most its inhabitants already converted to Catholicism.

Shortly after and from there on the Canary Islands became a hub for travels from and to the New World and with that an extremely wealthy place with a lot of trade happening there. Spaniards emigrated to the islands for this reason and became the majority themselves as the decades passed. Those Spaniards heavily intermixed with the Guanches, causing a rather quick assimilation to the Spanish culture and by the 17th century their language and self-identity disappeared altogether. This interbreed and cultural intermix transformed them into Canarians.

The Canarians were never "colonized" since they were the colonizers themselves, and as such they were never subjected to forced labor or any deprivation of their rights as lawful Spanish citizens. The sheer majority of Canarians back then and even today identify themselves as national Spaniards and regional Canarians. Genetic studies shows us that just around the 12-22% of the genetic make up of today's Canarians is made of possible aboriginal ancestry, and this is a tricky thing: the % comes from North African ancestry, which is the rather confirmed origin of the aboriginals, but then the mainland Spaniards themselves also have a common heritage with North Africa with a 4-10% share in their DNA markers.

There isn't even an ongoing independence movement supported by more than 1000 persons neither, and most the people supporting such movements are not exactly doing it for having another identity neither, but for political ones. The island has no political issues in any case as it is an autonomous region within Spain and with that they can exert themselves most the executive power in the island and they also provide seats to the Spanish congress, so they are properly represented just like any Spaniard. Its population of around 2 millions isn't to ignore neither!

Note: Socially, the islands are connected to the mainland by TV, Internet social sites and so on. There is a constant refresh in the ties with the mainland as there is a constant influx of mainland Spaniards to the islands and of Canarians to the mainland as well.

Officially, they are "logistic spaces." Many of them are military bases, others are bomb test sites (despite being inhabited.) Some are there to support military base nearby, with indigenous economy primarily supported by military outposts nearby.

It's about power projection and protecting/perpetuating "vital systems security." Historically, states' primary concerns were about defending territory. Today, it's about perpetuating systems that are vital to a state's continued functioning and its ability to provide for its population. Some examples are procuring fuel, energy, movements of goods. The health and survival of a population now depend on a transational economic system that they "must protect." What used to be an economic cost (trade disruption from natural disasters, terrorist attacks, rogue state provocation, regime turning against liberalism, geopolitical conflicts...) is now treated as security threat (Cowen, D.,2010. A Geography of logistics: Market authority and security of supply chains. Annals of the Association of American Geographers, p.3).

It's less about "conformity" than capacity to project power and protect liberal trade policies. So they're not there just because. They are controlled because they continue to serve vital function in maintaining hegemony.

It's a difficult question to answer because it depends so much on the island. In general the Pacific Islands that remained colonies belong to either France or the US, both of which largely failed to decolonize after the Second World War. For the most part it's because the islands represented strategic holds- originally as naval coaling stations, but after the Second World War they became important air bases, and later as nuclear testing locations. More recently, their EEZs (exclusive economic zones) make them extremely valuable for the amount of ocean they provide a claim over and the future potential for the discovery of rare earth minerals or energy sources in the deep territorial waters.

Generally speaking none of them are really in the middle of the ocean- but I assume you're trying to express the distance many of them sit from the metropoles that claim them.

At any rate if you have any queries about Pacific claims, I'd be more than happy to drop back in, but by an large strategic value is going to be an important part of any explanation.