The Evil Within's bigger picture in terms of storyline involves a very shadowy organization (much like Umbrella Corporation). While little is actually said about it in the course of the game, you eventually realize that it is indeed, a thing. But as for how important they really are, you never get to know. Sebastian's storyline is a little too preoccupied with the mindgames that he is going through. That is where Juli's story comes in. The parallel events that happen to her fills in a lot of the narrative gaps that made the main game feel broken. In this regard, this DLC is pretty much a must-play if you truly want to enjoy the game.
What is The Evil Within: The Assignment?
The Assignment focuses on the story of Juli Kidman –who gets introduced pretty early in the story of the main game, but you barely get to see much of her there. The Assignment lays out the events that happen to her and why Sebastian only gets to see her much later in the original game. Her point of view is pretty interesting because her story is parallel to the main game –not set after it. So as you progress through the story, Sebastian and Joseph are going through their own thing (which is why playing the first game is important before playing this, so you know the context of what Juli is seeing from her end). But more than just providing players of different perspective, Juli's importance in the big picture is even more complex. Of course, we will not spoil any specific details of who she really is, but playing her side of the story makes a lot more sense than Sebastian's.
Gameplay wise, Juli gets the more sensible gameplay system. Her stealth mechanics have been improved and streamlined to a much better degree. It does have its own share of hiccups (not surprising considering how sloppy things feel when you are in control of Sebastian), so there is still a lot to be improved in terms of controls. Her combat skills have a lot less options, more often than not, Juli has no proper weapons so you have to make good use of bottles and other things in order to make it to where you need to go. At the same time, her gameplay has been designed to be a lot less demanding or stressful so playing through this DLC is the better experience.
One of the best things that stood out in The Evil Within is its amazing visuals. Sure, we are not big fans of the more grosser environments that the game has at some points, but that does not mean they do not look well detailed. The lighting alone is masterfully applied in order to bring out the best of each location. Mikami makes great use of shadows and textures in order to manipulate the feelings and emotions of the players and this is done to such an extent that the backgrounds pretty much count as a character in itself.
Juli's voice clips are significantly better than Sebastians, her one-liners and random dialogue makes a lot more sense and make her character feel likeable or at the very least, relatable. This is a solid contrast to Sebastian's exasperated and disillusioned mannerisms.
Little Changes, Big Difference
As expected, the two characters play very differently. Juli is not reliant on weapons, and the stages she navigates is reflective of this (unlike Sebastian, who should be armed to the teeth, but is subject to constantly hiding). The complementing design of the character and the environment that the player navigates makes The Assignment feel more whole than the original.
This Should Have Been Bundled
If The Evil Within was indeed a re-envisioning of Mikami's ideas for a survival horror game, then the dual playthroughs of Sebastian and Juli would not be unlike having to choose between Chris and Jill for Resident Evil. However, Juli's campaign was designed as an add-on instead of being a main part of a standalone title and so it is pretty short (it can be finished in about three to four hours). Bottom line, if you want to fully enjoy The Evil Within, then getting The Assignment DLC pack is a definite must have.