The real reason is that they have limited supply, and people want them. (people may not want bitcoin in the future btw) People always want gold and they want to gather as much as possible in their lifetime. It is the only solid asset which holds its true value through the centuries.
There are lot of reason why we should hold our bitcoin and bitcoin. Furthermore, both of them have limited supply and there is a possibility that their price will increase in the near future and it can be the reason to gain much profit. Holding will really help us to gain much profit and it is very effective.