FoS's best and only reason of existence is concluding Arcania's story. Because other than that, it does nothing.
After the cursed demon left the body of King Rhobar III at the end of Gothic 4, it started to roam and wreck havoc upon the city of Setarrif. A city which was in war with Myrtanian forces, but since the demon becomes a common enemy for both sides, Rhobar orders you as some sort of agent to save Setarrif and destroy the demon once and for all.
Judging by its length, ( less than 5 hours no matter how slow you are ) FoS has good amount of story in it. ( For example, disagreement between Rhobar forces about helping Setarrif or fighting it ) and it's not as straightforward as you might think it is. But as mentioned before, Fall of Setarrif is just there to finish Arcania's story as fast as possible and you really shouldn't expect anything more than a conclusion.
One of the features mentioned for the add-on before its release was the fact that you could play as NPCs from the franchise ( Lester and Gorn ) and experience a part of the story from their point of view in a flashback sequence when they are explaining what they did to the nameless hero. This you can do. But it's extremely disappointing. Not only these parts are extremely short ( you can't even save your game ) but also, unlike it was mentioned, there's no unique skill set for the characters. Only the way they swing their weapon is a bit different.
New environments, items and enemies ( few as they are ) hardly feel or function different than their Gothic 4's counterparts and it's safe to say nothing of significance was added in this add-on.
One thing that FoS does better than Gothic 4 is the humor. There are 2 or 3 humorous moments which definitely put a smile in your face. (e.g. stupid daughter of Ethorn VI and they way nameless hero feels about saving her )
Overall, Fall of Setarrif does not feel different than Gothic 4 and it really doesn't look like a separate experience from that game in any way. It has the same graphics, voice actors, soundtracks, game-play structure and overall, flaws and advantages. Can't say it is an unnecessary add-on, since it's an answer to the cliffhanger ending of Gothic 4. But it's pretty obvious it could have been done a lot better. Promises behind FoS were not big, but even those small promises were not kept. Definitely the worst title in the whole franchise. Only play it if you were really interested in Gothic 4's story.