That's part of its whole failed "oppression" thing, where its separated into multiple "acts" with time skips in between to simulate the growing not-racial tension and changing city. Which again, good idea that was incredibly poorly done.

In general in RPGs we move from location to location. Modern games have the issue more, that you cannot backtrack between acts and are stuck on the new continent or whatever.
DA2 did basically the same - except you didn't move in space between acts but in time. Which is fine in general.
But it leads to people revisiting the same map multiple times. Which is also okay, since other games can deal with having the same cave layout for multiple caves. Yet DA2 combined the thing. Same layout for multiple caves and revisting those locations. With not enough variation over the time axis (unlocking a new area in an old map, ...).

The wave system is also a nice idea. It makes sense when you infiltrate the thieves den that thieves come running and join the fight over time.
But again - they overdid it. Open world ambush where the enemy would throw everything at you at once? Reinforcements. It also doesn't make sense when ancient constructs, undead or elementals do the same thing.

(which DA2 fails it, as you get one shot super quickly and have literally only one healer available for extended combat sections).

I modded the game on my first playthrough and kept my mage sister in the party. Companions were already boring that I don't felt I missed out one dialogue.