Independent troops (sometimes called Indies) are generic unit types that occupy most of the map at the beginning of the game.

Most are generic humans (such as Light Infantry or Heavy Cavalry) along with some commanders (such as the generic Commander or Barbarian Chief). More rarely provinces contain a mix of more exotic unit types; for instance, a province might contain Amazons, Pegasus Riders, and Crystal Sorceresses.

These same unit types can (generally) be recruited from a province. For instance, once you conquer the above province, you can recruit Amazons, Pegasus Riders, and Crystal Sorceresses from it.

Furthermore, the province defense in a province without a fort is generally made up of these same unit types.

Generally, indies are not as efficient as national troops, and you won't recruit many. There are some exceptions, though.

Scouts: Indie Scouts can do almost all the things your national scouts can, without taking up fort turns.

Mage diversity: Some indie mages (like Shamans and Crystal Sorceresses) are reasonably efficient and can provide access to paths that your national mages do not. In particular, nature access can be gotten from "tribal" indie types, which allow you to recruit Deer Tribe Shaman, Jaguar Tribe Shaman, Wolf Tribe Shaman, and the like.

These mages are generally not as efficient as national mages. But they can be very worth recruiting if they provide access to capabilities you otherwise would not have.

The rare useful indies: Some indie troops are actually worth recruiting. Notable among these are Crossbowmen (in Late Age where they are common), Hoburg Crossbowmen (very efficient), and maybe Zotz. These are typically useful to fill holes in national rosters; for instance, Crossbowmen will be more useful to LA Atlantis (who has no ranged troops) or Bogarus (whose troops are terrible) than to LA Man (who has a variety of very good ranged troops in its national roster).

Rare mages: Some magic sites provide access to efficient, high-end mages – for instance, Sage or Adept of the Iron Order. These are technically indies (since they are not part of your national roster), but are really in a class of their own; almost all of them are worth recruiting.

When expanding, it is useful to prioritize provinces containing poptypes that will give you access to mages you want. For instance, Shamans are very useful on nations that do not have astral access or high nature access. Since they are in provinces defended by Lizard Warriors, if you are one of those nations, you may want to make sure you get to a lizard province first to get Shaman access.