La Résistance is the DLC that made espionage a serious mechanic in Hearts of Iron IV, and along the way rebuilt France and gave Spain the depth it's known for today. Years on, it's still the expansion that most changes how a long occupation plays out.
What New Content It Adds
The pack splits its additions between two rebuilt focus trees and a layer of espionage and occupation mechanics that show up in any game, whether or not France or Spain have anything to do with it.
Nations: France, Spain, and Portugal
France's focus tree gets rebuilt from the ground up and 
Spain's expands massively — the Civil War gained all the depth it has today here, with separate focuses for the Nationalist side and the Republic. 
Portugal is also in the pack, though with a much smaller role.
Mechanics: Espionage and Occupation
The big contribution is the espionage layer: infiltrate agents into other countries, steal their tech, sabotage their industry or army, and decrypt their radio codes to get a peek at their war plan. On the occupation side, it adds support for resistance movements in occupied territory and the option to set up collaborationist governments instead of annexing outright.
Is It Worth It?
If you want to play Spain or France properly, it's close to mandatory — a big chunk of those trees' identity comes from here. And even if you never touch either, espionage and collaborationist governments show up in any long game with occupied territory. Of the major expansions, it's one of the ones that bleeds into games that have nothing to do with France or Spain.