In short, Yes.
Fiber is necessary for a lot of digestive processes.
One of the major downsides to the ketogenic diet is having to actively look for sources of soluble fiber.
Soluble, or Insoluble. That is the question...
There are two primary types of fiber we eat, soluble and insoluble.
Insoluble Fibers = ZERO Calories
Vegetables, especially leafy greens, are heavy in insoluble fibers. Such fibers are super hard for your body to digest, considering the fact that they don't dissolve in water. They, therefore, tend to pass right through your digestive tract.
Because of this, insoluble fibers are believed to have little to zero caloric load.
Soluble Fibers = 2 Cal/g
Soluble fibers are a bit trickier.
These fibers promote healthy bacterial growth, remove toxins and waste from the digestive tract, and result in better...uh... "movements".
They are found in things such as legumes, grains, nuts/seeds and some vegetables. Soluble fibers are partly digested due to being a bit easier on the gut to deal with. However, they are digested only partially, since they are too difficult of a carbohydrate to breakdown.
As a result, these carbohydrates only account for roughly 2 calories per gram, of which do not effect blood sugar. Think about that, they're carbs that don't count against your ketosis. Plus, they make you regular, and are super beneficial health-wise.
Fiber on Keto?
So, now that our preliminary science lesson is done, let's talk about where fiber fits in on a Keto diet.
Too Much Fat is Still a Bad Thing
If you didn't eat any leafy greens on a keto diet, your body would probably become really deficient in the essential vitamins and nutrients it needs to function normally.
So, sorry, but being "keto" is not a pass to eat nothing but full-fat bacon all day...
Furthermore, you'd get... errr.. "clogged"... by the lack of materials eaten to create a "movement". Without any sort of carbohydrate/protein, your body would basically only digest fatty acids, which aren't ideal for creating a healthy BM.
Believe me, you DO NOT want to be constipated while going through keto flu...
Protein Without Fiber?
Then there's the fact that while on a ketogenic diet, you are also still eating a smaller proportional balance of carbohydrates and protein.
The latter is important - the amino acids found in proteins can be converted into glucose (the enemy in a ketogenic diet), when there's an abundance of them in the diet and little-to-no carbs. Such is the state your body goes into during a ketogenic diet.
By eating fiber, you're basically curbing the digestion of the protein in your keto diet, slowing the potential process of its conversion into glucose. You're also telling the liver that there are some carbohydrates available, although they are not great for energy. The added dose of fiber prevents your body from trying to convert protein into sugar, but also doesn't kick you out of ketosis like regular carbohydrates would.
What to Look For
Keep it simple. When reading labels, look for your grams of fat to equal or outweigh the grams of non-fiberous carbohydrates and proteins. Remember, a gram of fat equates to the same as energy as 2.25g of carbs or protein (9:4cal). However, when you eat fiber, the proportion goes heavily in favor of the fat (9:2cal, if even that much).