Value and it's underpinnings act in very bizzare ways, and people might assign value to something just because other people do as well. Regarding Bitcoin, one of the reasons people assign value to it is because it offeres a way to keep track of value in a way that is almost impossible to manipulate. Cyclical reasoning right there. That property is not unlike gold, but besides gold also offeres some extra advantages (and disadvantages) like being able to transfer it across the world in 10 minutes or pass it across borders without any hassle.