Arms Against Tyranny puts the Nordic countries on the map for real, but what people actually remember this DLC for are the two mechanics that change how any nation runs its military industry and its arms exports.
What New Content It Adds
The pack splits its additions between five Nordic focus trees and two industry mechanics that show up in any game with even a minimally developed economy.
Nations: the Five Nordic Countries
Finland, 
Sweden, Norway, Denmark and Iceland get their own focus trees with real depth, in an area of the map that was basically pass-through territory before this.
Mechanics: MIOs and the International Arms Market
Military Industrial Organizations (MIOs) group your equipment manufacturers into consortiums that improve with use, with specific bonuses depending on what type of equipment they produce most. They add a layer of production progression that didn't exist before — it's no longer enough to just build factories, now it also matters which consortium runs them and what it's specialized in over time.
The international arms market makes buying and selling equipment to other countries far more direct than the old licensing system, and arms trade becomes a real source of income or diplomatic leverage instead of a side hustle.
Is It Worth Buying?
If you already have La Résistance or No Step Back and enjoy economic management, this is the natural add-on — MIOs and the arms market show up in any long game, wherever you play.