Some are saying the purpose is, "To expand influence. To do statecraft. To grow power." And yes, you're right. That is the hand wave that BioWare uses to gloss over the lack of a more immediate motivation.
I mean, that's the over-arching purpose. But you can have a sense of moment-to-moment purpose if you roleplay.
Take my last playthrough.
I was a Qunari mage. I just wanted to close the stupid breach and get the hell out of there, at first, I'm no Andrastian. But then I saw some stuff, and I started to feel bad for the people in the Hinterlands. Initially, I went to the Hinterlands to find Mother Giselle because I was told I needed to make the Inquisition known so I could get an audience with factions. She was so reasonable about magic that I stayed to do extra quests because I cared about the refugees plight. Maybe I did the other quests I ran into along the way, maybe I didn't, depending on my whim, but the primary "story" of the area is the refugees and I can help them - because it's right, because it spreads the Inquisition, because I hate war - or not help them. It's not required. It's optional. That's part of the point.
I went to the Fallow Mire to find my missing soldiers. Because I care about my people and nobody fucks with the Inquisition. Also, they challenged me directly, and I don't back down from a challenge. Pride, y'all. I closed rifts because demons pouring out of rifts is bad for Thedas, I found notes by an apostate and investigated it (or not, in some playthroughs).
There are reasons, but yes, you have to actually roleplay. There is also reactivity. People in the Hinterlands say happier things, and people at Skyhold talk about the refugees, and so forth, if you clear out certain things and do certain quests. You get agents. Etcetera. It's not required for end-game, but that doesn't mean it "doesn't matter" or "I have no reason to do it." Mechanically, it gives you XP, Power, Influence, and story wise there is some reactivity. (Could there be more? Sure. I hope there is next time. It's a good start, but I could see it improved even more, absolutely.)
I don't get the "I don't know what's going on" issue because every single zone gives you multiple ways to figure out what's going on. The Orlesian Civil War is mentioned dozens of times before Wicked Eyes, Wicked Hearts, at least. Maybe people who don't know about the books (I've read Masked Empire) don't realize it, but even in those zones, people will tell you what's going on. There are certain aspects you have to read missives for, etc, but that is a good thing to me, not a bad thing. I like when I can actually be a part of discovering the story.
This sort of story exposition likely could have been enhanced by cutscene discussions after the event (say, with any of the 3 advisers) but being able to dig it up yourself along the way is a good, subtle way to reveal aspects of the plot rather than being beaten over the head with plot background (some of which your character should not yet be aware of) at the beginning of each area.
I mean, more advisor cut scenes would definitely be good and maybe it would help those who don't like the reading. (To me, having me read it isn't lazy - it's a different type of storytelling - but I guess people don't like to read. It is cheaper and allows for more combat if written, instead of a full-on cutscene. Voice acting and animation cost a lot compared to text. But I'd rather have more story, and some of it be text, than only get what can be voiced.)
Take for example the Venatori searching the tevinter ruins in the western approach. It was just sortof there and you only find out about it because you try to get rid of some darkspawn, that apparently harrass you troops a little. Which agains throws up the question why i have to do it when i have an entire castle full of troops nearby but whatever.
They could have, for example, put it together with the one ruin with the time magic gone wrong in it. While you travel somewhere, a group of venatori spawns and attacks you with time magic, as long as your in the level range for the western approach. You then find some magical artifacts on them that reveal that they were found in the tevinter ruins in the wester approach. When you reach the western approach you find out that the venatori closed of the entrances and you have to find another way, by capturing the nearby keep as a powerfull outpust. This would also give you a reason to capture that keep besides "Because It's there".
Well, you know about the Venatori in the area before the Darkspawn, because you capture the Keep from them.
Then, there is a Darkspawn problem at the Keep.
So, you try to fix that problem (which requires multiple steps) and eventually find the reason the Darkspawn are pouring out of the area is because the Venatori were excavating.
To me, all of that made perfect sense, and the use of giants was a nice little surprise. (Though you find a note about it early in the dungeon, if you search everywhere, before you see the Giant - I saw the Giant first.)
I love the Silent dungeon as well.
To me, those areas had great story, lore, and interest. I thought they were very well done and they were some of my most intriguing moments in the game.
As to a "reason" to capture the Keep. You're given a reason already to hate the Venatori, so why wouldn't you capture any stronghold of theirs you came across? I mean, if you have a reason not to....then don't. That's the great thing about this game. So much is optional and you can play it different ways and still complete it.
That's great, but barely any of those objectives matter end-game. Thus making me say.. "Why am I doing this?"
I do get this point of view. I just don't agree with it per se. I don't think everything has to matter end-game - if it does, then you just have a linear story, not an open world. It would be cool if more Power = Something later or if the War Table quests did more for reactivity. But I don't want to be forced back into a linear story like DA:O, personally. I much prefer this form of storytelling.