Why is there sand in tape dispensers?

Without the sand the dispenser would be very light. The lighter the dispenser the less friction it well have between it and the surface it sits on. If this were the case when one would try to pull off a piece of tape the dispenser would be more likely to slide along the surface instead of sitting still and allowing the tape to easily roll off the roller. They could have just bulked up the dispenser by making it solid instead of hollow or perhaps made it out of something that is heavier than plastic. All these suggestions though were probably too expensive when compared to adding sand to a hollow body dispenser. One will find that everything in the engineering world really comes done to cost. Also the heavier tape dispenser can serve as a paper weight. Hope this answer totally fulfills your question.