Fiber is an important part of your diet. It provides a sort of roughage for your gut that helps to provide body to the substances moving through your system. When you get too little the system becomes sluggish and excesses left behind in the digestive tract can cause an imbalance between probiotics and yeasts. While these are necessary in their proper numbers to digest foods, if they get out of balance they will cause problems ranging from pain and bloating to diarrhea and infection.
Prunes are well known for their fiber value topping the chart at 15g per cup.
Blueberries are about average at 3.5g per cup but have the added benefit of preventing urinary problems.
Keep in mind that while a healthy fruit snack can be a good source of fiber, there is much more fiber in the skin of many fruits. Eating the fruit whole is much better than drinking the juice.