You have access to all of your recruitable, troops from the outset but some are not necessarily a good investment from turn one because of their cost. This is especially true of commanders, because you can only hire one per turn, and some are absurdly expensive.
Take Abysia, for example. Our "worst" unit is, relative to everyone else, very strong infantry - with a variety of equipment - each of which costs 20 gold and about 27 resources to hire. We also have some better units which cost about 60 gold and 40 resources to hire. Now, on the first turn you only have 400 gold to spend, plus - and this really matters for Abysia - only about 80 resources. So I can hire two of my best troops, or I can hire three of my slightly worse, but cheaper ones, and can then either pay for a better commander type right away, or save up to try to buy our best commander type (costs 440 gold) in turn two.
So even in the home province, where everything is unlocked from the outset, there's a lot of potential variety in strategy and approach. For example, as I mentioned above, even though they cost the same Abysia has 4 different infantry types: flail without shield, axe without shield, axe with shield, and axe with morningstar. Each of those is subtly different from each other, and each has their potential uses.
The real variety, however, comes from two places: independent provinces and summoning spells. Many of the best units in the game are only available through summoning rituals that can only be cast after a lot of research and by appropriately high-level mages. Sometimes, however, the most powerful of those spells might lie down different research paths, so you have to decide ahead of time which one you're going to target. Or, more commonly, a really good summoning spell will lie down one path, but a really good enchantment of some other kind will lie down another path.
Again, with Abysia as an example, "Contact Scorpion Man" - a spell unique to Abysia - lies at Conjuration level 8 (close to the end of the path), but the stuff that lies further up in the conjuration path isn't nearly as useful as, for example, some of the spells midway up the Enchantment and Evocation paths. So what do you, as Abysia, do?
Similarly, while most nations have better troops than are available in independent provinces, each province you capture gives you the chance to hire troops from it that differ from your own. As Abysia, I have no archers, so if I want to hire some, I need to hire them from independent provinces. Many nations lack mages of proficient in certain magic types, so independent provinces are sometimes the only way for them to gain access to that magic. Finally, almost every land-based nation in the game has a hard time getting into water provinces without either 1) powerful magic or 2) conquering provinces that have amphibious troops available for hire.
There's a none-too-short answer to your question. Probably any video you sought out gave you the same answers, but, hey, differentiated instruction and stuff... Some people prefer to read.